[Tex/LaTex] How to make tikz multipart node parts have uniform size

Question

When making boxed nodes in TikZ, a common requirement is that the nodes be of equal size. Because TikZ automatically adjusts the size of nodes to fit the contents of the node, this is usually done by setting a minimum size for the nodes, and setting the inner sep to 0.

How can I do the same thing for the parts of multipart nodes? As the following example shows, setting a minimum size for a multipart node only sets the height (as described in the manual). So how do I make each part be the same size as my non-split nodes?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.multipart}
\usepackage[margin=1in]{geometry}
\parindent=0pt
\begin{document}
\tikzset{
every node/.style={draw, minimum size=4ex,inner sep=0pt},
split/.style={rectangle split, rectangle split parts=2,draw,inner sep=0ex,
   rectangle split horizontal,rectangle split part align=base,minimum size=4ex}}
\begin{tikzpicture}
\node at (0,0) {A} ;
\node at (1,0) {a} ; 
\node at (2,0) {X} ;
\node at (3,0) {j} ;
\node at (4,0) {};
\node[split] at (5,0) {}; 
\node[split] at (6,0) {X\nodepart{two}j};
\end{tikzpicture}

\end{document}

Answer 1

I updated my answer with the idea from Caramdir's comment. This method is used in the tutorial "Diagrams as Simple Graphs" of the pgfmanual.

Update With text width=4ex it's better to use Polgab'method : align=center instead of \hfil ...\hfil

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.multipart}
\usepackage[margin=1in]{geometry}
\parindent=0pt
\begin{document}
\tikzset{
every node/.style={draw, minimum size=4ex,inner sep=0pt,textstyle},
split/.style={rectangle split, rectangle split parts=2,draw,inner sep=0ex,
   rectangle split horizontal,minimum size=4ex},
   textstyle/.style={text height=1.5ex,text depth=.25ex}}

\begin{tikzpicture}
\node at (0,0) {A} ;
\node at (1,0) {a} ; 
\node at (2,0) {X} ;
\node at (3,0) {j} ;
\node at (4,0) {};
\node[split,text width=4ex] at (6,0) {\nodepart{two}}; 
\node[split,text width=4ex] at (8,0) {\hfil X\hfil\nodepart{two}\hfil r\hfil};
\end{tikzpicture}  

\end{document}