I am having a quandary about this myself.
I am in TV Production at university, where we have all Macs that use Final Cut Pro. This semester I'm finishing up right now has been all tape-to-tape editing, except that on the final news package, we're allowed to use nonlinear. I've got my entire project finished in a borrowed FCE 4 before most others have even finished shooting their footage, much less edited anything.
Next semester is all nonlinear, so here is my dilemma. Go ahead and buy FCE4 now, and risk FCE5 showing up in the next month or two, or wait. Adding to the mix is I really would not mind buying Adobe Production Premium for not really all that much more than FCE4, and get not only Premiere, but also Photoshop, Flash, etc. The one gotcha with THAT, is that I've been using the 30 day demo of Premiere, and for some reason it doesn't want to import from my DV cam (a Canon Elura 100 - anybody know why? It seems like Premiere CS4 is really limited on what dv cams it supports.).
What to do? Buy FCE4 now while I need it, wait on FCE5, buy Adobe Prod. Premium for $419 (academic disc.) are the choices.