Differences across borders in table

Your table is too big for a4 paper, so the situation is much worse in the beamer frame. You need to restrict its width to textwidth and enable to break text in cells into more line. For this is handy to use tabularx table:

documentclass{beamer}
usepackage{ragged2e}
usepackage{booktabs, tabularx}
newcolumntype{L}{>{RaggedRighthspace{0pt}}X}

begin{document}
begin{frame}
    begin{table}
    scriptsizelinespread{0.84}selectfont
    setlengthtabcolsep{4pt}
begin{tabularx}{linewidth}{@{} 
                >{hsize=0.8hsize}L
                >{hsize=0.75hsize}L
                >{hsize=1.25hsize}L
                >{hsize=1.2hsize}L @{}}
    toprule
textbf{Author} & textbf{Field} & textbf{Question} & textbf{Method}  
    midrule
M. B"{o}hme et al. (2018) & Migration & Predict international migration with online search keywords & GTI trends   
    addlinespace[2pt]
D. Garcia et al. (2018) & Gender inequality & Using Facebook advertising data to infer gender inequality & Fb marketing API    
    addlinespace[2pt]
L. Timothy et al. (2019) & Health & Is income inequality harmful for health? & Geospatial Big-Data   
    addlinespace[2pt]
R. Chetty (2020) & Intergenerational mobility & New insights on social mobility in the US & Geospatial information + administrative tax records    
    addlinespace[2pt]
M. J{ae}ger & E.H. Blaab{ae}k (2020) & Education inequality & Inequality in learning opportunities &  Access and takeout materials from public libraries  
    addlinespace[2pt]
S. Su at al. (2017) & Health & Socio-economic status and access to healthy food & Geo-big Data on four types of mobility   
    addlinespace[2pt]
J.L. Toole et al. (2015) & Macro-economics predictions & Unemployment tracking and predictions & Mobile-phone data   
    addlinespace[2pt]
P. Brown & M. Souto-Otero (2018) & O-E-D relation & Investigating the E-D relation in UK & Job Adverts from Burning-Glass Labour Insight  
    addlinespace[2pt]
L Sloan et al. (2015) & Social-class analysis & Obtaining socio-demographic characteristics from Big-Data & Twitter users' description 
    bottomrule
end{tabularx}
    end{table} 
end{frame}
end{document}

If you like add to frame a frame title or table caption, you need further reduce table font size or extend table over two slide. For example with use of the xltabular, however, its features is not the same as at other documentclases.