As DNJ always says "When you're out in the trenches every day" you want the lightest, easiest to set up and transport keyboard available that does the job you wish. When I was a younger man, which was about the same time John Paul Jones joined the Navy, I hauled around a PSR-5700, a monster about the same size as the T4, but tipped the scales at 51 pounds. Back then, 51 pounds was a piece of cake. Then the PSR-500 came along, it was 30 pounds lighter, provided better sounds, more options and features and I fell in love with it. That 5700 was gone in a flash. I've played a T2 and a T5, both of which sounded great. However, when I picked them up and placed them on the keyboard stand, I immediately knew they were too heavy and bulky for everyday players that perform in several different locations. I also got to play an S-970 for a couple hours - WOW!
If you're a play at home player that only plays out once in a while, the T4 is just fine. But, the S-970 has a huge number of fantastic features, incredible sounds, it's easy to transport and set up, and as DonM, (a very wise man) said, a $5 adapter cable solves any issues with the mic connection.
Good luck on whatever you decide upon,
Gary