documentclass[11pt]{article}
usepackage[table]{xcolor}

definecolor{myblue}{RGB}{78, 126, 222}
usepackage{multirow}
usepackage{geometry}
usepackage{tabularx}
usepackage{booktabs}
usepackage{ragged2e}

begin{document}

begin{table}[ht]
    centeringsetlength{extrarowheight}{3pt}
rowcolors{1}{myblue}{}
        begin{tabularx}{textwidth}{>{cellcolor{white}} c >{cellcolor{white}}c>{RaggedRightarraybackslash}X>{RaggedRightarraybackslash}X}
            toprule
            & Inner index 1.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            multirow{-2}{*}{Outer index 1} & Inner index 1.2 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            midrule
             & Inner index 2.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            multirow{-2}{*}{Outer index 2} & Inner index 2.2 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            midrule& Inner index 3.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.2 & multicolumn{2}{>{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelaxRaggedRightarraybackslash}X}{Text data placeholder. May be long enough that it needs to be wrapped. Text data placeholder. May be long enough that it needs to be wrapped.}\
            &Inner index 3.3 & multicolumn{2}{>{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelaxRaggedRightarraybackslash}X}{ Text data placeholder. May be long enough that it needs to be wrapped. Text data placeholder. May be long enough that it needs to be wrapped.}\
            multirow{-2}{*}{Outer index 3}&Inner index 3.4 & multicolumn{2}{>{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelaxRaggedRightarraybackslash}X}{ Text data placeholder. May be long enough that it needs to be wrapped. Text data placeholder. May be long enough that it needs to be wrapped.}\
            &Inner index 3.5 & multicolumn{2}{>{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelaxRaggedRightarraybackslash}X}{ Text data placeholder. May be long enough that it needs to be wrapped. Text data placeholder. May be long enough that it needs to be wrapped.}\
            &Inner index 3.6 & multicolumn{2}{>{hsize=dimexpr2hsize+2tabcolsep+arrayrulewidthrelaxRaggedRightarraybackslash}X}{ Text data placeholder. May be long enough that it needs to be wrapped. Text data placeholder. May be long enough that it needs to be wrapped.}\
            bottomrule
        end{tabularx}
end{table}

end{document}