If you haven’t been working on this new technique for more than a few weeks, then sometimes that is the reason for having trouble. It takes time for the 4th finger to develop flexibility once you are in the right position. My suggestion is continue to focus on the fundamentals and don’t change the two most important things–keeping the hand back and the hand high.
There is a good chance you are grabbing the fingerboard too tight still as I see that bad habit all the time. You should be able to easily slide the hand up and down the neck with ease. If you are clamped to one area, then the overall tension might be what is making it hard to stretch far enough for the 4th finger.
Any chance you could post a video or picture? I can really give you some good insight then.