Changes to the previous WIP:
The big heart in the first half of course 4 appears to be the best option because of how much time I spend close to it anyway. Grabbing it costs only 26 frames, compared to the 64 or so it takes before it becomes collectible. There's another heart further in the tunnel that's also worth looking into.
There's a new method in the course 3 revisit to get a return instruction in memory without having to go way above the top of the screen. By breaking a block while there's still a jet flame, ALL of the flame's data is copied into the AAF0 slot, instead of just what gets left behind after it unloads.
As expected, this new setup saves several seconds on the minimap. The run's not officially sub-4 yet because I'm still relying on the accessible debug mode to set my coin and heart count, but I've been spending the past few days investigating how much I can do to manip coins without losing any more time. My estimated maximum is generally hovering around 63, but I can still lose a few frames and still make the same frame rule. We'll see if that's enough.
As for the 36 coin/1 heart setup, it doesn't look viable. My best attempt at crossing the lava pool with only 1 bird kill loses about 150 frames, and I'll lose another 80 or so not being able to despawn the course 3 skull door.