There is absolutely zero frame difference between the 3 and the 4, so if the components are correct for a 3 there is nothing to worry about. It's a number on the seat tube that hardly anyone will even know what it means, those that do won't give a rat's arse, and you won't even really notice unless you fixate on it.
Unless your buying the bike to advertise "hey look at the fancy expensive bike I ride", the number is meaningless.
If you've managed to actually find one I'd grab it. Put another sticker over the number if it bugs you (or it may be over the clear, in which case it will pull off with a little working on your end, but I suspect it's sealed on).