Try this:
Z=10
X=4 * Z
PRINT X
if you get 40 then the issue most likely has to do with printing an intermediate result via the command line; I vaguely recall a bug like this in a BASIC interpreter form 40+ years ago though I don't recall the details.
Then try this:
10 Z=10
20 PRINT Z*4
RUN
If this also work then the issue is pretty much isolated to how the command line is being handled.
-Neil