There really is no comparing the FE2 with the OM4. I love Nikon gear, and my old black F is my favorite SLR camera, but for a light, compact, and feature-packed camera, the OM4 is nearly impossible to beat. Others have mentioned the meter, which even the latest digital wonder cameras cannot compete with, but there is also the brighter finder, a diopter adjuster (the FE requires screw-in diopter lenses), and, with the T cameras, the ability to use the F280 flash at any shutter speed. The OM is also smaller, and a bit quieter than the Nikon.
As for lenses, I find the plain-Jane Zuiko 50/1.8 lens to be my favorite. Nikon lenses may be more sturdy, but they are also more heavy, and don't seem to focus with the same smoothness/preciseness as the Olympus lenses. Also, I prefer the quality of the images from the Zuiko 35, 24, and 21mm lenses over their Nikon equivalents.
But, after all that I have written above, I actually shoot an FM3A. I feel that the OM system is superior in several ways, but I find Nikon equipment more suitable to myself.