I agree with lederhosen, it must be something with your trim not being spot on, so you have your arm/shoulder slightly different than on the other sizes.
If you have a fixed bar on the 4 m2, easy to adjust the rearlines, if not, still easy, but you have to shorten the rear lines or lengthen the fronts at the kite - or reverse, depending on "where" you are off
How far away from your CL have you set the bar sweetspot?
I have shortened the rearlines 10 to 15 cm compared to the default setting - so I ride with bar quite far away from me, least stress this way for me.
But this is personal - just dont ride with the bar all the way in of course, gives you a very stressful arm/shoulder position when carving.
The 4 m2 doesnt have much bar pressure, so thats not it
I would say the 4 m2 is a lot less tiring than all bigger sizes, as you need such gentle inputs only.
Also as one said, dont ride toeside for longer tacks - can be very bad for both arms in fact.
No problem if you never has a problem - but IF you do, this can enhance the problem.