To me, short of going the dedicated upper, the Colt M4 is the closest in looks and feel, although the inner guts a completely proprietary.
In the pro's side, it very cool, weighs about the same as the real thing as it's made of metal unlike the S&W/M&P which is polymer so it's very light. When you pick it up it really feels like and M4.
If you live in a state other than CA, you also get the benefit of a threaded barrel, so you can replace the flash hider with other nifty gadgets.
With a bunch of accessories on it it can be made into a pretty facsimile of a M4. There also 4 variants to choose from, from a M16 rifle to the M4 OPS carbine.
The only CONs are the no functioning bolt release (whatever lame brain came up with that idea should be flogged), and a heavy trigger pull (about 8lbs.)
Anyway, I do like mine, and very accurate, at least up to 50 yards, which is all I can test it on, since the local shooting range doesn't allow rimfire on the 100 yard range