I'll jump in, So first congrats on your layout, good luck with the new one and hope you'll post some pics.
I believe the biggest difference in jumping up in size from the 4x8 is you will get wider, more realistic curves with smoother running trains. You can run also run slightly longer equipment (if desired). From what I've seen the biggest jump is to the 8x8 size layout. Lots of action, two levels, lots of scenery options.
Best of luck and enjoy the process