Distribute columns evenly in table Latex

I would use a simple, five-column tabular environment and expend some effort on making the legend intelligible and informative.

documentclass{article}
usepackage{amsmath,ragged2e,booktabs}
begin{document}

begin{table}[ht!]
caption{Explanatory regressionsstrut}  label{tab:OOPSexpweek}

Centering
begin{tabular}{@{} l cccc @{}}
toprule
Variables & multicolumn{4}{c@{}}{OOPS excess returns} 
cmidrule(l){2-5}
& 1 & 2 & 3 & 4
midrule
$r^{}_{mathrm{S&P 500}}-mathit{rf}$ & 0.2825  
& [13.10%]      
$mathit{JUMP}$ & & 0.9031  
& & [7.50%]     
$Delta mathit{RV}$ & & & 0.6697 
& & & [8.30%]   
$Delta mathit{VIX}$ & & & & $-$0.0005 
& & & & [74.20%]  addlinespace
$bar{R}^{2}$ & $0.009$ & $0.019$ & $0.011$ & $-0.002$ 
bottomrule
end{tabular}

justifying

medskipnoindent small
Table ref{tab:OOPSexpweek} presents the coefficients---$p$-values, 
calculated with Newey-Weststandard errors, are in square brackets---and 
the adjusted $bar{R}^{2}$ statistics from four explanatory regressions. 
The dependent variable is the weekly OOPS excess returns, and the 
independent variables are listed in the column `Variables'. **~and *** 
indicate significance at the 5% and 1% levels, respectively. The 
estimation period is January 2012 and December 2019; the number of 
observations is~416.

end{table}
end{document}

---

Addendum to address the OP's follow-questions.

• To place the legend above rather than below the tabular environment, simply copy and paste the legend material to a location between caption and begin{tabular}.

• To spread out the tabular material so that its width equals textwidth, I suggest you employ a tabular* environment.

documentclass{article}
usepackage{array,booktabs}
begin{document}

begin{table}[ht!]
setlengthtabcolsep{0pt}
caption{Explanatory regressionsstrut}  label{tab:OOPSexpweek}

medskip
begingroupsmall
This table presents the coefficients---$p$-values, 
calculated with Newey-West standard errors, are in square brackets---and 
the adjusted $bar{R}^{2}$ statistics from four explanatory regressions. 
The dependent variable is the weekly OOPS excess returns, and the 
independent variables are listed in the column `Variables'. **~and *** 
indicate significance at the 5% and 1% levels, respectively. The 
estimation period is January 2012 and December 2019; the number of 
observations is~416.
parendgroup

medskip
begin{tabular*}{textwidth}{@{extracolsep{fill}} l cccc }
toprule
Variables & multicolumn{4}{c}{OOPS excess returns} 
cmidrule{2-5}
& 1 & 2 & 3 & 4
midrule
$r^{}_{mathrm{S&P 500}}-mathit{rf}$ & 0.2825  
& [13.10%]      
$mathit{JUMP}$ & & 0.9031  
& & [7.50%]     
$Delta mathit{RV}$ & & & 0.6697 
& & & [8.30%]   
$Delta mathit{VIX}$ & & & & $-$0.0005 
& & & & [74.20%]  addlinespace
$bar{R}^{2}$ & $0.009$ & $0.019$ & $0.011$ & $-0.002$ 
bottomrule
end{tabular*}

end{table}
end{document}