If your 4th finger is collapsing, it means your left hand shape is sub-optimal. Most likely from inadequate rotation of your forearm/wrist.

The most optimal hand/arm shape in all circumstances is that which allows you to play parallel 3rds. If that is a high challenge...then put down your bow. In 1st position set:

B-on the A string (1st finger), and then additionally set 3rd finger G on the D string. This will likely require you to adjust your handshape a bit, which is what we want to fix.

THEN add in:

C-on the A string (2nd finger), and finally A on the D-string (4th finger)

Resulting in 1/2 on the A string, and 3/4 on the D string all at once. This will likely require further hand/arm/wrist adjustment... Keep 1/3 down and lift 2/4. Then put 2/4 down again. And back and forth. Then do the opposite. keep 2/4 down, and raise 1/3. This will likely be a bit of a challenge, DO NOT press keep your finger action light....it is a natural tendency when doing this kind of work to tighten up, which we don't want. This handshape will net you a nicely curved 4th finger by bringing your hand in proper alignment with the neck of the violin---so long as you don't tense up.....if you tighten up, that 4th finger will collapse.