Update! ... Kind of.
Most of World 4 is now done. I found improvement in 4-2, by taking an ice block to run into the small plant instead of slowing down to fireball it, so from then on will be redone (that's why I stopped in the middle of 4-F2). It's probably for the better, because I was getting repeatedly screwed by the randomness with the hammer brothers. I want to be able to fight the one before 4-4, so I can use firepower, and I also was able to get a flower card on 4-6, but hammer brothers got in my way, so I had to wait for a mushroom card. Maybe on the redo, things will work out better for randomness.
I may also use the musicbox I have here. World 5 isn't too congested with hammer brothers, and World 6 is so huge and multiple-path-having that I think the musicbox may benefit me the most here.
Highlights:
4-4 turned out pretty well, although the Lakitu acted a little different than when I just randomly tested this strategy a while ago, so maybe I will gain a few more frames when I redo it.
4-6 has a lot of lag. I haven't tested against waiting for the plants to lower into the pipe (you can't fall between them because there are invisible blocks on the right side).
4-F2 may benefit from getting superspeed on the donut lift with the fire flower. Either way I need the flower because there isn't one on the airship (there are 2 leaves instead). I tested that strategy roughly; using a leaf vs. fire flower are just about the same speed (within 10 frames or so). However, the fire flower here will speed up the Boom-Boom battle, so it wins in this case.
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Edit: Run redone, link updated.
In 4-2, I have to turn to the left with the ice block or the fireball that gets the first plant out of the way will disappear. I performed a glitch at the end of the level that takes time because I end up waiting until that particular frame to get the card in order to manipulate the hammer brothers as needed and also because I need a mushroom card.
Hammer brother fought earlier and with firepower (about 20 frames faster).
4-4 improved slightly by running off the edge instead of jumping, which for some reason saved 3 frames. Also got a flower card here.
I tried three different techniques in 4-6: the original, glitching through a pipe, and what I ended up with. The bounce over the second pipe has invisible blocks, so I cannot move to the right any sooner than I do.
147 frames faster when entering the second fortress, which is also finished and improved slightly.
