I think the bottleneck with 4-5 trills is the fact that finger 5 is shorter than the 4th, and they're more or less dependent on each other's position (especially the 4th finger).
What helped for me was to practice tremolos involving 4th and 5th fingers alternating and acquire the speed from the rotating wrist, concentrate on conditioning myself to allow the 5th finger dominate the rhythm and pushing the 4th finger further up the key so that the wrist is angled [to right] horizontally (so the "distance" both fingers had to "travel" down on each keypress were about equal), and then move towards the 4-5 trill (taking the thumb out but maintaining the wrist movement and trying to progressively compensate it with fingers' motion independently).