The Roland FP-4 is the older model. If the price quoted is for new, it is a good deal, IMO.
The FP-4 shares the same non-SN (SuperNATURAL) sound engine with the FP-7. The FP-4F has the newer SN engine, which avoids the looping of sustained notes, velocity-layer jumps and other artifacts which hamper realism. The SN models tend to allow for better expression due to an excellent action/sound connection. The FP-4F also has a newer and more sophisticated action than the FP-4, although it has been noted that it is perhaps more "sluggish" as well as heavier-weighted.
That said, the older model still has a good sound and is capable of being played expressively, despite the lighter action. However, it perhaps doesn't have the timbre range of the SN models. The FP-4 has a user-selectable/editable FX section that the newer "F" models lack, and can also remember more MIDI parameters
(EDIT: see my post below in reply to anotherscott about MIDI functionality). In addition, its electric piano patches are arguably superior to the FP-4F. Its speakers are not the best, but then, from what I can gather, neither are the FP-4F's.
I would say that the decision to purchase the FP-4 over the FP-4F would depend to a large extent on the kind of music you play. I wouldn't suggest purchasing it for classical repertoire, but it copes pretty well towards the jazz/pop/rock end of the spectrum. To give you an example, I am rehearsing a jazz standards set on the FP-7F but will be gigging with the FP-4 due to its lighter weight.
Whatever you decide, I think that the Rolands (older and newer) are quality instruments that will serve you well.