What tips do you all have at using 4th finger?

I'm having a few problems with it. Playing on a 16.5" viola, it's a bit of a stretch. I always hit the note, however, it's pretty flat when I compare it to the open string (the note is recognizable, as let's say a D, but it's a flat D when compared to the open D). I can stretch a little, to hit it spot on. I am able to hit 4th finger a little sharp as well.

The thing is, having to "stretch" does start to tire me out after a while. The two worse strings to do so is C and G. D and A, I usually get it spot-on. G-string requires some more practice, and C string is a harder stretch. I do use all 4 fingers to press down the string in regards to playing 4th-finger.

My teacher said that it was alright if I slide the third finger a little in order to help with the stretch. I do find it easier to press the 4th finger if my third finger is playing the high-3 fingering. 

I can't apply pressure at the left side of my pinky, like I do the other fingers due to the stretch.

My pinky is able to push the C-string down while remaining curved, for those who are wondering, however in the right position, it's a bit harder. In a low-4 position, it's easier.

Advice helps.

Thanks.