Aligning multicolumn environment in tabular to the top of cell

Question

I have a table with 4 columns and 4 rows, in the last row of which I'm using multicolumn to have cells 2 columns wide. However, since one is taller than the other, for some reason they are automatically aligned to the middle (vertically speaking) of the cell, not the top. I have tried p instead of l in the column designation, putting in whitespace (positive and negative), searching on TeX.SX, but I haven't found a solution. Here is a MWE:

\documentclass{standalone}
\usepackage{amsmath, tikz}
\begin{document}

\renewcommand\arraystretch{1.5}
\begin{tabular}{l l|l l}
sheaf space & & stack space & \\\hline
$U\to {\mathcal F}(U)$ & a group & $U\to \widehat {\mathcal F}(U)$ & a groupoid \\
$O(X)^{op} \to \text{Set}$ & open sets to groups & $O(X)^{op}\to \text{Grpd}$ & open sets to groupoids \\[5pt]
\multicolumn{2}{c|}{
\parbox{7cm}{\raggedright
if $s_i\in \mathcal F(U_i)$ and $s_j\in \mathcal F(U_j)$ such that $s_i|_{U_i\cap U_j} = s_j|_{U_i\cap U_j}$, then there exists $s\in \mathcal F(U)$ such that $s|_{U_i} = s_i$ and $s|_{U_j} = s_j$.
}} &
\multicolumn{2}{c}{\parbox{7cm}{\raggedright
if $s_i\in \widehat{\mathcal F}(U_i)$ and $s_j\in \widehat{\mathcal F}(U_j)$ such that there an isomorphism $\varphi_{ij}:s_i|_{U_i\cap U_j} \to s_j|_{U_i\cap U_j}$, then there exists $s\in \mathcal F(U)$ and isomorphisms $\varphi_i:s|_{U_i}\to s_i$, $ \varphi_j:s|_{U_j}\to s_j$ such that the following diagram commutes:
\[
\begin{tikzpicture}
\node (si) at (0,0) {$s_i|_{U_i\cap U_j}$};
\node (sj) at (3,0) {$s_j|_{U_i\cap U_j}$};
\node (s) at (1.5,1.5) {$s$};
\draw[->] (si) to node[above] {$\varphi_{ij}$} (sj);
\draw[->] (s) to node[auto,swap] {$\varphi_i|_{U_i\cap U_j}$} (si);
\draw[->] (s) to node[auto] {$\varphi_j|_{U_i\cap U_j}$} (sj);
\end{tikzpicture}
\]
}}
\end{tabular}
\renewcommand\arraystretch{1}

\end{document}

This is the output:

I would like the text in the bottom left cell to be aligned to the top of its cell, but I can't figure out how to do it.

Answer 1

\parbox is vertically centred by default use \parbox[t] or better use

\multicolumn{2}{p{7cm}|}{ 

instead of

 \multicolumn{2}{c|}{\parbox{7cm}{