OK, was able to try a few out. The OTK4 is too small, especially at the hip area. I got in a Large T11T Tillett and it felt really good. Also tried an X-seat (which appears to be the same as an IPK or IMAF seat) in size 4, and it felt good, just a little more snug. I ended up purchasing a Tillett T11T LARGE from Stu at MDG Karting. I may have gotten the X-seat instead but it sounds like Tony Kart may have worked with Tillett on the T11T (or the other way around) and also my brother has the exact seat and chassis, so we want out equip as close to the same as possible, and adding a different seat just adds one more variable.
Which means I have been running an XL with padding this whole time, when I probably should have been in a Large. Hopefully this shaves 2 seconds off my lap times ;)