I think one needs to be careful trying to stretch the pinky, or any fingers. It isn’t hard to injure the fingers more when trying to manipulate the stretch. Stretching the fingers takes time, and forcing it will lead to injury. I know it’s hard, but just keep trying to persevere. Others here use Chinese balls, putty and other gentle exercises can help with loosening up the fingers, thus giving you a more relaxed stretch. I used to just try and hold the index finger on the first fret, and the pinky on the forth and hold the position until I start to fatigue. If that is too much of a stretch, move down the fretboard and have your pinky on the 12th fret, and the index on the 9th fret. The distance will be shorter, and you can eventually start to move down the fretboard. Also, a shorter scale guitar will help. Most of my guitars are 24.9″ scales opposed to the standard 25.5″ scale. The difference in fretting is significant.