The Nikkor 55mm f/2.8 AIS Micro is excellent. I love mine.

The F4, excellent rep aside, does seem like a bit much for this scenario. None of Nikon's pro film SLRs fall into what I'd call "discreet." My F100 is probably the trimmest and it's just as big as my DSLRs. For general urban use, I prefer my FE2.