Nothing wrong with different models/skins so long as they don't give you an advantage IMO. I really don't see what's wrong with getting a better look M4. Also, the server consistency is just for world models AFAIK. IF you delete the world models then you can still see the new models/skins of the weapons you're holding, just the ones other ppl are holding will look original (that goes for weapons ont eh ground as well).
To delete weapon world models delete any inconsistent files starting with
w_ in the following directory:
SteamSteamApps[[[username]]]counter-strike sourcecstrikemodelsweapons
I know this works, cos I just played in a PCW against BIATCH and they were enforcing consistency on the match server. I had a couple of models that were "inconsistent" and couldn't get on. As soon as I deleted all the w_models (time was of the essence) the server let me on just fine and I still had my fancy new models/skins in game