"Things DO NOT happen overnight and you will have to try them for at least a month to see whether they are suitable for you (in my opinion).
Others may disagree, but I seriously doubt that you can see that a mouthpiece is better for you from one practice session, as there will be no time for changes/ adjustments to be made for the change of mouthpiece."
That was my experience when I started using the SM3, two years ago. I was previously using the Bowman BB1. They didn't give me the sound and gave I wanted for my euph. which I just bought at the time. I went through a couple of months of rebuilding my range a bit and just getting air. It was a bit nerve whacking at the time as I was in the middle of putting together my senior recital at university but that's why they evented summer holidays for

. The 3 at times for me, almost gave me a too big of a sound but that might have to do with the players I was playing with the concert band and brass band that I was playing at the time.
I have used the SM4 as well. The 4 is a touch less taxing getting air through the horn but to me, the low register doesn't respond as well as the 3. If you like the shilke 51D and find the SM3 a bit too big, the 4 will work nicely for a person.
For my trombone, I use the shilke 51D, I don't have too much problems switching mouthpiece wise on either of the Mead mouthpieces I use and the shilke on trombone.
In the end, it just gets down to the persons lips, tastes etc. what may work for one person may not work for another.