You don't quite have 4" in the rear. You're looking at more like 3.7". It's generally a good idea to keep the front and rear travel pretty close to each other, otherwise you make some pretty big compromises in handling.
This doesn't exactly answer your question but;
I recently put a Noleen fork on my '01 FSR, and tried it in both 75mm and 100mm lengths, with both 75mm and 90mm on the rear suspension. When the fork is set to 3", the bike turns very quickly and climbs the best, the 4" fork really eats the bumps on fast downhills, with a small compromise in climbing (I dropped my stem height to compensate). 100mm fork and 90mm rear seem to match pretty well, so if you ran a 120mm fork, climbing will be more difficult (front wheel lift), and handling will be a bit slower, which means it should go downhill just fine. I think if the fork is travel-adjustable, go for it, but if it's a fixed 120mm fork, you might not like the handling/climbing. Folks have said the FSR frame seems strong enough at the head tube for longer forks, most breakages seem to be in the rear end on '00 and earlier anyway.
As some folks in the Specialized forum have mentioned, you *could* get a BETD link to increase your rear travel to 110+mm which would balance the 120mm fork better.
I have a Psylo SL 80-125mm on my 99 FSR-XC with a 100mmn rear link, and I usually ride it with about 100mm in the front. However, if the climb is long and steep, I drop it down. The 125mm in the front only comes in handy in long downhills, but there is no way I can climb anything remotely steep with that much travel in the front, even if locked out.
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