The problem on OS 4.0 is - as HammerD says - the lack of RAM (which, incidentally, is a limitation of the CPU slot, not Phase5's or Hyperion's fault). Yet OS 4 supports swap files, which would be dead handy, just not on the Classic platform where it's most needed!
I wish Hyperion would either do OS 4.1 for the Classics or even just a patch to add virtual memory - it'd make the Classic much more useful.