Wa'alaikumussalam! Biasa je tu at 4 months, dia tengah explore how to move kan, so ada je tangan yang tersangkut or lenguh when they sleep on their side, nothing to worry about lah. A few of my mum friends went through this juga, and it usually sorts itself out once dia can roll properly. Just make sure pillows and extra bedding jauh dari area dia tido, and maybe try positioning dia slightly different so the arm ada support, but honestly, this is one of those things na if you're worried, best to check with your paed just for peace of mind!