I believe a 21° hybrid would be a good 4 iron replacement.

I’m going for a fitting next week to gap from a T300 7 iron. Including a T 300 6 iron as a possible option. I have an 818 23 hybrid set to 25 degrees to substitute for the 6 iron when it is in the time-out corner. But, yeah, you can’t empirically compare lofts to substitute hybrids for irons.

Noting I can barely hit a 6 iron, my suggestion, besides getting fit, I’d suggest you keep an open mind on whether to get the TSR-1 or TSR-2 hybrid. If you can hit the 5 iron with enough club head speed, you might like the TSR-2 better than the -1. I have always had the -2 versions as the LH options in the TSi-1 LH options were limited. We’ll be looking at both lines to fill gaps. …May the best club win!

21° hybrid is between a 3-4 iron, 18° a 2 iron.

I have a 21 adjusted to 22 as a 4 and an 18 adjusted to 19 as a 3. That gaps well to my 16.5 Sim HL 3 wood…..only non titleist club in the bag.

I have a set of T-300's. My set goes to the 5 iron (23*) then I have a U505 at 22*. Then a TS1 Hybrid 3 at 18*. The gap between the 5 iron and utility club doesnt bother me. Different situations for each of these clubs.

Settled to go with my T300 6-GW
TSi1 10° Driver
TSi1 15° 3 wood at 16.5°
TSi1 20° 7 wood
TSi1 23° Hybrid
TSi1 26° Hybrid
54° wedge
58° wedge
Putter

I have the same set up and carry a 16.5° TSi2 3-wood, and 19° 910h and a 21° TSi2 hybrids.