Regression output using siunitx - alignment / column width problems

Thanks agains for your helpful suggestions.

Finally, I found have another way to get what I want using dcolumn. To make it work properly, I had to add small spaces in the regression output between the regression coefficients and the standard errors (subption incelldelimite(":") in esttab).

% Code

documentclass[letterpaper,12pt]{article}
usepackage[utf8]{inputenc}

usepackage{adjustbox}
usepackage{booktabs,subcaption,amsfonts,dcolumn} 
newcolumntype{d}[1]{D..{#1}}

begin{document}

begin{table}[!h]
  centering
  caption{xxxx}
  label{tab:4}
begin{adjustbox}{max width=6in}
    input{Table_4.tex}
end{adjustbox}
\[3pt]
begin{minipage}{6in}
footnotesize{Note: Hierarchical logistic regression with country fixed effects and random intercepts by election. Standard errors in parentheses. Significance levels: $^{+} p<0.1,  ^{*} p<0.05$, ^{**} p<0.01$, ^{***} p<0.001.$}
end{minipage}
end{table} \


% Table
{
defsym#1{ifmmode^{#1}else(^{#1})fi}
begin{tabular}{l*{3}{d{10}}}
hlinehline
                        &multicolumn{1}{c}{(1)}&multicolumn{1}{c}{(2)}&multicolumn{1}{c}{(3)}\
                        &multicolumn{1}{c}{shortstack{\SUV}}&multicolumn{1}{c}{shortstack{\Yes}}&multicolumn{1}{c}{shortstack{\Interaction}}\
hline
SUV                     &-0.38:(0.09)sym{***}&-0.49:(0.14)sym{***}&-0.46:(0.19)sym{*}  \
SUV X Contact: Yes      &          :         &          :         &-0.33:(0.16)sym{*}  \
Yes            &          :         &0.30:(0.08)sym{***}&0.55:(0.13)sym{***}\
SUV X Contact: No       &          :         &          :         &0.02:(0.26)         \
No             &          :         &-0.21:(0.24)         &-0.21:(0.36)         \
Age                     &0.09:(0.01)sym{***}&0.09:(0.01)sym{***}&0.09:(0.01)sym{***}\
Age^2                    &-0.00:(0.00)sym{***}&-0.00:(0.00)sym{***}&-0.00:(0.00)sym{***}\
Female                  &0.01:(0.03)         &0.01:(0.03)         &0.01:(0.03)         \
Education: Primary      &0.31:(0.11)sym{**} &0.30:(0.11)sym{**} &0.30:(0.11)sym{**} \
Education: Secondary    &0.62:(0.12)sym{***}&0.61:(0.12)sym{***}&0.61:(0.12)sym{***}\
Education: Post-Secondary&0.67:(0.11)sym{***}&0.67:(0.11)sym{***}&0.67:(0.11)sym{***}\
Education: University   &1.13:(0.14)sym{***}&1.12:(0.14)sym{***}&1.12:(0.14)sym{***}\
Income: Quintile 2      &0.16:(0.07)sym{*}  &0.16:(0.07)sym{*}  &0.16:(0.07)sym{*}  \
Income: Quintile 3      &0.30:(0.06)sym{***}&0.30:(0.06)sym{***}&0.30:(0.06)sym{***}\
Income: Quintile 4      &0.51:(0.05)sym{***}&0.51:(0.05)sym{***}&0.51:(0.05)sym{***}\
Income: Quintile 5      &0.52:(0.10)sym{***}&0.52:(0.10)sym{***}&0.52:(0.10)sym{***}\
Small/Mid-Sized Town    &-0.10:(0.06)sym{+}  &-0.10:(0.06)sym{+}  &-0.10:(0.06)sym{+}  \
Suburb of a Large City  &-0.11:(0.06)sym{+}  &-0.11:(0.06)sym{+}  &-0.11:(0.06)sym{+}  \
Large City              &-0.11:(0.08)         &-0.11:(0.08)         &-0.11:(0.08)         \
Close to a Party        &0.98:(0.05)sym{***}&0.97:(0.05)sym{***}&0.97:(0.05)sym{***}\
Efficacy                &0.34:(0.02)sym{***}&0.34:(0.02)sym{***}&0.34:(0.02)sym{***}\
Constant                &-4.05:(0.25)sym{***}&-3.98:(0.23)sym{***}&-4.01:(0.27)sym{***}\hline
Country-Year Variance   &0.04:(0.01)sym{***}&0.06:(0.03)sym{+}  &0.06:(0.03)sym{+}  \
hline
Country FE              &multicolumn{1}{c}{Yes}         &multicolumn{1}{c}{Yes}         &multicolumn{1}{c}{Yes}         \
N (individuals)         &multicolumn{1}{c}{75183}         &multicolumn{1}{c}{75183}         &multicolumn{1}{c}{75183}         \
N (elections)           &multicolumn{1}{c}{64}         &multicolumn{1}{c}{64}         &multicolumn{1}{c}{64}         \
hlinehline
multicolumn{4}{l}{footnotesize }\
end{tabular}
}

https://i.stack.imgur.com/dDG2P.png

Here is my suggestion:

documentclass{article}

usepackage{siunitx}
usepackage{booktabs}


defsym#1{ifmmode^{#1}else(^{#1})fi}

begin{document}

begin{table}[!h]
  centering
  caption{XXX}
  label{tab:4}
sisetup{table-space-text-post=sym{***},
         table-align-text-post=false, 
         table-format = -1.2(3)}
setlength{tabcolsep}{0pt}
begin{tabular*}{textwidth}{@{extracolsep{fill}}lSSS}
toprule
                           & {(1)}               & {(2)}              & {(3)}                \
                           & {IEF}               & {Contact}          & {Interaction}        \
hline
SUV                        & -0.38 (9)sym{***}  & -0.49 (14)sym{***} & -0.46 (19)sym{*}    \
SUV X Yes                  &                     &                     & -0.33 (16)sym{*}    \
Yes                        &                     &  0.30 (8)sym{***}  &  0.55 (13)sym{***}  \
SUV X NO                   &                     &                     &  0.02 (26)           \
NO                         &                     & -0.21 (24)          & -0.21 (36)           \
Age                        &  0.09 (1)sym{***}  &  0.09 (1)sym{***}  &  0.09 (1)sym{***}   \
Age²                       & -0.00 (0)sym{***}  & -0.00 (0)sym{***}  & -0.00 (0)sym{***}   \
Female                     &  0.01 (3)           &  0.01 (3)           &  0.01 (3)            \
Education: Primary         &  0.31 (11)sym{**}  &  0.30 (11)sym{**}  &  0.30 (11)sym{**}   \
Education: Secondary       &  0.62 (12)sym{***} &  0.61 (12)sym{***} &  0.61 (12)sym{***}  \
Education: Post-Secondary  &  0.67 (11)sym{***} &  0.67 (11)sym{***} &  0.67 (11)sym{***}  \
Education: University      &  1.13 (14)sym{***} &  1.12 (14)sym{***} &  1.12 (14)sym{***}  \
Income: Quintile 2         &  0.16 (7)sym{*}    &  0.16 (7)sym{*}    &  0.16 (7)sym{*}     \
Income: Quintile 3         &  0.30 (6)sym{***}  &  0.30 (6)sym{***}  &  0.30 (6)sym{***}   \
Income: Quintile 4         &  0.51 (5)sym{***}  &  0.51 (5)sym{***}  &  0.51 (5)sym{***}   \
Income: Quintile 5         &  0.52 (10)sym{***} &  0.52 (10)sym{***} &  0.52 (10)sym{***}  \
Small/Mid-Sized Town       & -0.10 (6)sym{+}    & -0.10 (6)sym{+}    & -0.10 (6)sym{+}     \
Suburb of a Large City     & -0.11 (6)sym{+}    & -0.11 (6)sym{+}    & -0.11 (6)sym{+}     \
Large City                 & -0.11 (8)           & -0.11 (8)           & -0.11 (8)            \
Close to a Party           &  0.98 (5)sym{***}  &  0.97 (5)sym{***}  &  0.97 (5)sym{***}   \
Efficacy                   &  0.34 (2)sym{***}  &  0.34 (2)sym{***}  &  0.34 (2)sym{***}   \
Constant                   & -4.05 (25)sym{***} & -3.98 (23)sym{***} & -4.01 (27)sym{***}  \
midrule
Country-Year Variance      &  0.04 (1)sym{***}  &  0.06 (3)sym{+}    &  0.06 (3)sym{+}     \
midrule
Country FE                 & {Yes}               & {Yes}               & {Yes}                \
N (individuals)            & {75183}             & {75183}             & {75183}              \
N (elections)              & {64}                & {64}                & {64}                 \
bottomrule
multicolumn{4}{l}{footnotesize Note: Significance levels: sym{+} p<0.1,  sym{*} p<0.05, sym{**} p<0.01, sym{***} p<0.001.}
end{tabular*}
end{table}

end{document}

It appears that you may not be aware that the ways that measures of "uncertainty" are displayed in tabular formats in the natural sciences is quite different from the ways employed in the statistical reporting of regression results. siunitx is geared, by default, to the natural sciences' ways of doing things. In regression tables, though, standard errors are usually displayed in separate rows, below the corresponding coefficients.

Anyway, I would recommend that you place the standard errors on separate rows.

Another comment: Since all numbers already appear to be rounded to two decimal digits, the options round-mode = places and round-precision = 2 don't seem to be needed.

documentclass{article}
usepackage{siunitx}
sisetup{input-open-uncertainty  = ,
         input-close-uncertainty = ,
         table-space-text-pre    = (,
         table-space-text-post   = sym{***},
         table-align-text-pre    = false,
         table-align-text-post   = false}
defsym#1{ifmmode^{#1}else(^{#1})fi}
usepackage{booktabs}
begin{document}

begin{table}[!h]
centering
caption{XXXstrut}
label{tab:4}

begin{tabular}{@{} l *{3}{S[table-format = -1.2]} @{}}
toprule
& {(1)} & {(2)}     & {(3)} \
& {IEF} & {Contact} & {Interaction}\
midrule
SUV    &-0.38sym{***}&-0.49 sym{***}&-0.46 sym{*}  \
       & (0.09) & (0.14) & (0.19) \
SUV$times$Yes & & &-0.33sym{*} \
       & & &  (0.16) \
Yes    & & 0.30 sym{***}& 0.55 sym{***}\
       & & (0.08) & (0.13) \
$vdots$ \
midrule
Country FE          & {Yes}    & {Yes}   & {Yes}   \
$N$ (individuals)   & {75183}  & {75183} & {75183} \
$N$ (elections)     & {64}     & {64}    & {64}    \
bottomrule
end{tabular}

medskipfootnotesize 
Note: Significance levels: $^{+} p<0.1$,  $^{*} p<0.05$, $^{**} p<0.01$, $^{***} p<0.001$.

end{table} 
end{document}