Subjective experiences:
FED 4 is better made, especially the lens. I find the match-needle meter easier to use since the scale on the FED 5 meter is rather small. The FED 5, on the other hand (presumably you were meaning a 5C, since that has the meter) has a self-resetting frame counter (dubious if that's better or not), hot-shoe for flash and also has a much brighter VF, with crude parallax-correction framelines.
Both have surprsingly accurate and useful meters: they aren't badly affected by stray light, as you might think from the positioning. Size and weight are pretty much the same on both models.
I think the quality went downhill over time and that's the main reason the 4 is better made. My 5C has a terribly made I-61 L/D which even wobbles slightly on its mount but actually works perfectly well. In terms of overall usability there's little to choose and they are both dirt-cheap so you won't lose much money if you get a bad one or if it gets dropped etc! They are both pretty chunky and not overly beautiful but they do take fine pictures.
Two small bits of advice - it took me a while to find the rewind-release on the FED 5 - it's actually a case of pressing the collar around the shutter button and it re-sets when you wind on next. The other thing is that, like all other FEDs, flash sync works correctly ONLY at 1/30 and NO OTHER speed, even "B".
