I have bought both se4 & se5 and I say get se5 now before it go's off the shelf like so many games do.
I play both still (with mods) the modders should know i bug them to UPDATE thier mods (hint hint)
I've been playing se4 WAAYYY back when it was ver 1.49
thats when i was hooked and i'm hooked on se5
I do have se3 somewhere on a cd but it's the demo ver 1.17
So yes get se5 now and learn the UI (bugs and all)
(but never give up on the old games, they are the one's we all learned from)