I think on the website it even states that in order for you to get the best possible gains you need a 90mm TB or something like that. So, if you’re not going with a 90+mm TB go with the 3.5. I would say for the extra $$$ you are getting a better look with the 4 since it’s a nice single piece. I went with the 4 b/c I had the 90mm TB on the way at the same time. If you ever plan and getting a bigger TB get the 4 or try to see if someone is selling 1 in the 4 sale section. Save yourself some money.
Found it from the site "Track results have shown very little results from the 3 1/2" to 4" BWoody Jeep SRT-8 using the OEM 80mm Throttle Body, Largest gains will be achieved with the 90mm throttle body."