Right now, 4 1/4 feels thin, and after some hitting, my handsl and forearm are feeling sore. I'm just not used to it. However, when going back to my main racquet, a modern Head at 4 3/8, it now feels kind of fat. I may go down to 4 1/4 and put a thicker overgrip on, or go with thinner grips on my 4 3/8. We shall see how I deal with this moving forward.
I'm not going to dismiss your issues with grip sizes - any issue with gear that becomes a distraction is a significant problem. If you think that a thinner grip on your 4 3/8" handle will fit you okay, maybe try to stick with that for a while and see how you settle in. Lots of variables at work here.
Not sure whether you're switching among racquets from the same brand or not, but different companies have their own particular grip shapes/profiles. Every Prince racquet I've ever used has always felt like the grip was smaller to me than the same size from let's say Wilson or Yonex. So a 4 3/8" from one company could feel about the same as a 4 1/4" from another.
Everybody has their own personal normal. One pair of racquets I use have 4 5/8" handles with heat shrink sleeves added on along with a replacement grip and an overgrip on top of that. They work great for me because I've used a 4 5/8" handle forever. And I'm a bass player, but I stick to 4-strings.
Forearm soreness can come from a few different directions. If a racquet doesn't have the same balance as the one that's your regular player, that can trick you into using too active of a wrist/forearm to leverage the strings to the ball - usually the case if that different frame is less head-light than your normal racquet. And if the handle is so small that you need to use extra gripping pressure to stabilize the racquet for some shots, that can be stressful, too.
If the strings in a racquet are too stiff for a particular player - yes, polys - that can also potentially bring on all sorts of irritation or even some injury. That's another topic though.
Even if you have the right handle size, one issue that can lead to extra stress from gripping too hard can be not having enough of a flare down at the butt cap. Without enough of that bump at the bottom of the handle, that can trick us into holding on much more tightly than is necessary to keep the racquet from slipping when we want to take a full cut at the ball. I ran into this issue myself a few years ago and it was a big relief when I added an extra layer or two of overgrip to build up that flare by just a little bit.
