355 tapered for all Titleist irons since at least the original AP irons

I have a set of modus 105t stuff pulls

355 taper tip for Titleist
Guess it depends on why you’re not hitting 4&5 well. Modus are a little stiff in the handle, and I’ve had to keep the speed/intent up to get the most out of them - especially longer irons. Don’t be afraid to get after it.
Given 105 is lighter than 120, you’ll likely get more speed/distance on good strikes which won’t be a bad thing. But the bend profile of 105 is different than 120 - so ball flight might be different. May just take some range time to get them dailed in if you switch.
I just switched from 105Xflex to 105 Stiff. Mostly for the winter, but hoping to not work as hard and increase consistency.
Good luck on your journey!