It depends on your definition of "powerful". If "powerful" means "more things can be connected", then yes. If it means "louder or more features", then no.
The T8s does not have more power than the T4s -- neither device has an amplifier that powers a speaker, they just mix and process the sound (tone, reverb, effects, etc.) and send the signal out to its destination. Whatever you have connected to your T4s won't get any louder just by switching to a T8s.
The T8s is (more or less) an 8-channel version of the T4s, except that it has four more channels, two more aux outputs, and one of the two sets of main outputs uses XLR jacks rather than ToneMatch ports (the other set is 1/4", same as the T4s.) It has a separate port to use the included Tonematch power supply.
Otherwise, the operation and menus are basically the same, except for adding menu options for the extra inputs and outputs.