Please... not even a contest. RE4 hands down for two very good reasons:
1) RE4 is wwwwaaaayyyy better (either version)
2) RE4 is cheaper (you might be able to get RE4 for GCN & PS2 together for the price of FEAR).

Also, depending on your PC, FEAR may not run that well... but if you can run it fine, you can disregard that. FEAR is good... but definitely not as good as RE4... not even close. For those of you that just played the demo only, let me just say the demo is REALLY good... as a whole, the experience the demo gives you leaves a much better impression on you than the entire game because is squeezes in everything cool about the game into like 15 minutes of action. I played the demo, and I was totally hooked. The full game is pretty much the same experience, but more drawn out, so it gets a bit more monotonous. RE4 is pretty much the greatest game ever... so yea.