My opinion, if you have the extra cash, grab the x4. If you don't, get the x3. Long story short. You may be able to unlock, but there is no guarantee. Also, some of those cores could be disabled due to defects. So you won't be guaranteed to get an x4 from the x3. Number one rule when buying parts, if you intend to keep it a while, buy the best you can then later on you don't have to upgrade as soon if you don't want to. That's my 2 cents.
P.S. Can't speak for the gaming or overclocking of the x3, but my guess is they are a decent gamer chip as a lot of guys seem to be recommending them for budget builds.
Most games use up to 3 cores but some use 4 or more so the 3 core will be better or as good for most current games but future ones will be using more cores. Aslo less cores normally means more overclocking room.