I have been playing on and off for many years, but mostly just strumming chords. I’m struggling with the stretch from the 2nd to the 4th fret as well. As someone else mentioned, sliding or using your pinky finger are both acceptable ways to play this until your fingers stretch and get limbered up. I currently have to take my index finger off the 2nd fret to reach the 4th fret with my ring finger, but as I practice it is getting easier. The main thing is to not get frustrated, keep practicing this, but work on other areas as well. Some aspects will come easier than others. Be happy for the things that work, and keep practicing on the things that need work until they fall in place.