The lofts aren’t as drastic as the T400 and the T350 4G has a forged face, better weighting, and for me, launches easier and works better the further off the fairway as the rough gets deeper.
It will be an individual choice. If the T350 doesn’t improve over the T400 in an outdoor fitting, stay with the T400. Most will likely prefer the T350. Or T250 Launch.

The T400s have a lot stronger loft. For example, the 400s had a 26* loft on the 7 iron while the 4G T350s have a 7 iron loft of 29*. For me it made it almost impossible to get the 400s sufficiently up in the air. I also found the heads to be a bit bulky and somewhat cumbersome. The T350s have performed quite well except I cannot get a high launch or enough spin out of them.

As a relatively slow swing speed, golfer, I really wanted the T400s to work out for me. I have a six and seven iron that I hit from time to time, but I just can’t make them perform well enough to want to fill out the rest of the set. Very excited about the new T250* Launch Spec irons that are going in the opposite direction with weaker lofts and more spin promotion. Just recently ordered these.

Big fan of the TSR1 line as well as the GT1s. I commend Titleist for giving moderate swing speed golfers options.