I've posted this on other sub 4 1500 threads but some simple but effective workouts are:
10 x 400 in 65 w/ 1 minute rest
10 mile run in 60 mins 5 min break to change into flats/ spikes. 1600 on track in 4:40 even paced.
4x800 in 2:15 running 35,35, 35, 30 for 200 splits. 3 min rest interval
One slightly more challenging workout is:
2 sets 3x400 in 63 w/ 30secs rest then 1 mile in 5 flat/ 5 minute set break. The mile is run 30 secs after last 400. 400 split can be adjusted as necessary but should be run at least 1 sec faster than current mile pr pace. If you do this workout, commit to it and don't cheat the rest or the mile interval.
Track workouts are just one part of the equations. Tempo runs, weights, pylons, rest, and nutrition are just as important if not more so. Track work is definitely necessary but the main training effect is mental rather than physical.
Running sub 4 for 1500 is all about being able to run 64 per 400 smoothly over the first 800 and then still run 64ish splits while digging in. With your solid but not top flight 800 marks do not fool yourself into thinking you can kick your way from slow pace to a 1500 PR. Most likely splits for a 3:59.xx 1500 are something like 63 flat,64.x,65.x, 47 low.
If you ran 4:26 last year and have been training consistently sub 4 is a very reasonable goal. Good luck-