Everything is the same as the 3SG, except one HUGE difference - I can run my Accucraft locos at scale speeds now.
I am totally battery powered. Those Accucraft units are geared very low, With a 3SG, I could run the 4449 GS4 at a max of about 40 mph, and that was with a 5S LIPO. With the 4SG, I can run a 6S HV LIPO in it and get about 26V and run that critter at about 65, much more realistic and fun.
So I'm a satisfied 4S user. I think if you go with a 4S, it will give you everything the old 3S would, but also provide the capability to get your speed way up there, especially with USAT and Aristo locos. Have to make sure you install the latest HC software too.
Rob