Do you have problems reaching to play 4th finger at times? Is the stretch uncomfortable or painful for your hand?
This is something that I've had to really work on over the years. Growing up playing violin, it wasn't an issue playing 4th finger. I have a medium sized hand, so no problem there. But during my years studying at the Conservatorium, I changed to playing viola... and a big viola! Since then I've had to constantly work at stretching out my hand and finding ways to reach 4th fingers, double and triple stops, and other difficult intervals. One teacher in particular gave me some very good advice for finding a way to comfortably shape the hand. And that was this:
Always place the 4th finger comfortably first, then stretch back to place the 1st finger.
Try it on your instrument at home... put the 1st finger down and stretch up, and see how far you can reach. Now try putting the 4th finger down and stretching back. Is there a difference?
It's very important whenever you are trying to stretch into uncomfortable positions to keep your hand soft! If you try to force your hand into a position, you can cause strains over time and injure your hand. Relax the hand as much as possible, stretch back softly, and experiment with how you can gently shape your hand to reach.
If your hand is relaxed, it should be able to find the best way to comfortably reach everything. Every hand is different, and will need a slightly different way to set. Rather than trying to force it into a position (that may work perfectly for another player), let it find a way comfortably and softly. It might not be the way you thought it should be, or what works for your teacher or other musicians. But it may be much better for you.
Once you have set your 4th finger and softly stretched back, take note of what part of your first finger is now pressing down the string (where on the tip of the finger). What shape is the 1st finger - how bent are the joints? How close is the knuckle of the 1st finger to the neck of the instrument?
After trying this a few times, shake out your hand out and see if you can return to the same hand position, placing down only the 1st finger. Now it should be easier to reach the 4th finger...