Sometime we have our own ideas about what we prefer. The purpose of going to a fitting is to get a club that gets the maximum performance, designed just for our swing and ball speed. I never second guess my fitter. He has always been right. Just my two cents worth. Have not seen the heads and no info on the sight to compare, but did come across the info on the internet:

"the TSR3 historically has had a more traditional pear-shaped profile that lowers spin and is often preferred by better players. I can see a movable weight in the sole just behind the face of the new GT3, which indicates the model is likely to spin less (as weight is positioned forward) like the TM Qi10 LS and cally AI Smoke Triple Diamond, it’s likely to also offer the opportunity to dial in ball flight shape a fraction too.

While for seriously high-speed players who want to eliminate spin the TSR4 has always been the brand’s lowest-spinning driver option. It appears the new GT4 which will replace the TSR has a very similar front fixed sole weight as its predecessor so it’s fair to expect a similar performance to the TSR4 with the addition of any new tech, Titleist has built into the GT family. "

GT4 is gonna be less forgiving for sure. You also don't get the surefit track adjustment.