If you're framing the ball too much with your PK, that might just be an issue with your technique or just that you're using a racquet that you're not used to yet. If the heft and balance of the PK is significanly different from the LM 4, it will probably be easier for you to put the ball in the heart of your strings more often with one over the other.
It's tricky to define exactly what makes any racquet "arm friendly" for an individual player, but adside from getting consistently good contact on the sweet-spot, it's usually smart to consider softer strings if you're using something on the stiffer side of the street. If you have any poly in your racquet now, try something else and see what happens. As far as I've seen, poly is usually harder on elbows than shoulders, but it could be worth a try.
Sometimes shoulder pain can result from "arming" the ball too much, but some shoulders object to a racquet that's too heavy, especially for hitting lots of serves. Your LM 4 isn't a terribly hefty beast, so I'd only worry about hurting your shoulder with that frame from over-swinging with it.
Soft frames will typically be kinder to elbows, but can also yield less "pop". If you think you need a frame with a lot of flex, you will also want it to have some extra weight so that it can deliver some power when you swing it.