I have noticed that the poses for v4 are a bit inaccurate for Sasha, especially in the arms... Is there any way to fix this?

You'll just have to fix them manually each time you use a pose, Sasha had different joint setup than default V4 plus all the ERC coding for the Sasha controls and enforced limits, etc. Of course once you've fixed a pose you can always save it back to your library (preferably as a new pose just for Sasha, as to not overwrite the original for V4).