String crossing in descending slurred passages is vulnerable to unevenness. There are two reasons. The rhythm of the bow only helps the 1st note of every slur. Also as we move from the E-string to a lower string the natural slope of the hand means that the lifted 4th finger moves towards a higher string (the B-string!) not towards the A-string.
Move under the violin with the whole arm and this will move the whole hand (especially 4th finger, which will drop first) to the new string. Put in a practice break before the string cross to concentrate on the arm swing.
Here Fintan plays 4th finger followed by open string, then 4th finger followed by 2nd finger. Of course also practice 4 1, then all fingers descending 4 3 2 1 0.
Scores - Left-Hand Frame Exercises - violin & viola