Your pump looks like a two stage but it is difficult to tell in your pics. I have a 3 1/2" ram which came off of another log splitter.
I've been told both on this forum and from friends that a 3 1/2" or 4" is all you need and anything bigger is overkill for most splitting.
If you have a good wedge and pusher design, this will make up for a smaller cylinder. 4000 psi is more than most splitters are set up for
so it sounds like you have plenty of pressure. If your cylinder cycle times are acceptable now, I'm sure you know that they will decrease a bit
when you go to a bigger cylinder. That could mean a pump upgrade but you only have a 5.5 hp so that might limit you a bit.
Page 33 in the surplus center catalog has some pump specs/hp guidelines.
https://www.surpluscenter.com/\CAT284\Catalog284.pdf