Ilford FP4 (w/o Plus) was discontinued in 1990. Treat it like you would any very old and expired film. It'll have a lot of base fog and you'll want to over expose it probably a lot. I'd run a test roll with frames ranging from ISO 125-25. You can probably find development times in old manuals or on the inside of the boxes (if you have them). The times will not be accurate but they may be a starting off point. I might also try the Plus times to see what happens. Another thing you could try is a 1% benzotriazole solution to reduce the base fog that will surely have built up over time. There is no real way to say what the condition of the film is, considering we can't know how it was stored. Deep freezer? Use it as new. Under the bed? Uh, see what happens.
I'm not sure what Ilford said about the differences per se. I'm guessing at the time they considered the "Plus" to be "Better". I doubt they'll be drastically different.
