Try hammers and pulls with each of the sets:
1, 4
2, 4,
3, 4
..where 1 refers to index finger, 2 to middle, 3 to ring and 4 to pinkie.
Most people find the 3 to 4 trills the most difficult and which need the most development.
I got a development on this exercise.
All fingers on 1 string, each fretting an adjacent fret. E.g. G string, frets 3, 4, 5 & 6.
Keep all fingers fretting... take one finger, do open string pull-off/hammer-on on the adjacent string above. E.g. All fingers still down, take pinky, do pull-off/hammer-on on D string.
Very tiring exercise because it isolates a finger and you really work it out. Mindless/brainless sit-in-front-of-tv exercise too.