Couple of things...
I agree with above. My 480s simply BLOW AWAY the 390s I used to own -- there was a night and day difference while using the same recorder through my transition from 390s to 480s (I owned both for a while too).
Next, I don't think anyone mentioned that another difference b/w the R4 and the R4 Pro is that the pro has trim nobs in addition to gain nobs (I think). Gives you much more control vs. the mic/line-in choice on the non-pro model. However, someone that owns a pro should confirm that.
Lastly, not saying you should go this route, but for what it's worth, I've been recording for a year or more on a stock R4 (no mods), and I've made some nice recordings. If you want some samples from the R4, my website (see sig) has a ton of different shows recorded with the R4 in 2-chan only mode, AUD+SBD Post-Matrixes, and AUD + spot mics or flanking mics (head's up though, some shows were AT943>ST9100>H120, but they are real obvious when you listen to them).