My experience with the 2 mouthpieces is that you won't really gain much, and you'll be out some money. The Yamaha 4c is a decent mouthpiece, however I do prefer the C*. It seems more comfortable to play the full range of the instrument and a bit more even in the tone of the instrument. The Yamaha 4CM is very very close to a C* in almost every way and a good backup for a C* for less money.
I had a student come to a lesson with his new Yamaha horn & mouthpiece and he was squeaking just awfully. The facing on his new mouthpiece had a nasty bump in the facing. I was able to fix it quite easily, but I have been cautious about recommending the Yamaha pieces since then. Their reputation is pretty good, but personal experience tells my they aren't perfect.
I read in another thread that you aren't a rookie musician, but that saxes are new to you. The 4C is fine for starters, you will most likely want a better horn before you outgrow the Yamaha piece. If you want to try a C* look for CM (custom) Yamaha. They can be found much cheaper than the Selmer and are 99.99995% as good.