MuseSounds are too quiet.
In a score I made with MuseScore 3, I had a harp instrument set to "piano." When listening to it in MS4 with MuseSounds, I realized that "piano" is barely audible, so I had to change it to mezzoforte. Also, some harp samples are much quieter than others, making it seem like it's occasionally missing notes (especially when playing in octaves). The celesta (MuseKeys) has a similar issue.
There are still tons of bugs with MS4, and I wish the developers had waited longer before releasing it. I'm definitely going to be staying with MS3 for a while, even though I love all of the features MS4 adds.