I'm interpreting your requirements as follows: (a) Column "A" should be half as wide as column "B", and together they should span the first half the available width; (b) columns "C" and "D" should be equally wide and span the second half; (c) the contents of all cells should be top-algned; and (d) the height of the middle row of cells is determined by whichever of cells "One" and "Six" is taller.
If this interpretation is correct, the following setup may be what you're looking for. The only shortcoming, in my view, is that the horizontal line that separates cells Two/Four from cells Three/Five isn't balanced automatically; depending on the contents of cells Two and Four, a \phantom
instruction may be needed, as is demonstrated in the code below.

\documentclass{article}
\usepackage{array,ragged2e}
\newcolumntype{P}[1]{>{\RaggedRight\arraybackslash}p{#1}}
\newlength\tablengtho
\newlength\tablengtha
\newlength\tablengthb
\newlength\tablengthcd
% Calculare widths of columns A, B, and C/D
\setlength\tablengtho{\dimexpr\textwidth-8\tabcolsep-5\arrayrulewidth\relax}
\setlength\tablengtha{\dimexpr0.6667\tablengtho/4\relax}
\setlength\tablengthb{\dimexpr1.3333\tablengtho/4\relax}
\setlength\tablengthcd{\dimexpr\tablengtho/4\relax}
\begin{document}
\noindent
\begin{tabular}{| P{\tablengtha} | P{\tablengthb} |
P{\tablengthcd} | P{\tablengthcd} |}
\hline
\multicolumn{1}{|c|}{\textbf{A}} & \multicolumn{1}{c|}{\textbf{B}} &
\multicolumn{1}{c|}{\textbf{C}} & \multicolumn{1}{c|}{\textbf{D}} \\
\hline
One One One One One One One One One &
\multicolumn{1}{@{}c@{}|}{%
\begin{tabular}[t]{P{\tablengthb}}
Two Two Two Two Two Two Two \\
\hline
Three Three Three Three Three \\
\end{tabular}} &
\multicolumn{1}{@{}c@{}|}{%
\begin{tabular}[t]{P{\tablengthcd}}
Four Four Four \phantom{Four} \\ % \phantom{Four} needed for balance
\hline
Five Five Five \\
\end{tabular}} &
Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six Six \\
\hline
Seven & Eight & Nine & Ten \\
\hline
\end{tabular}
\end{document}