If SMAC/X won't run under a Pentium 4, producing "Processor not supported" errors
then Edit the Alpha Centauri.ini file so that it contains the following lines at the start of the file:
[PREFERENCES]
ForceOldVoxelAlgorithm=1
If you still get cpu not support messages thereafter, just ignore them and click OK, the game will start.
This particular fix should be in the paper and read me documentation shipped with your game. It has been there for years, since before the Pentium 4, originally to deal with some non intel processors.