Steeping is a sealed bottle sitting for awhile out of sunlight to allow all the components to mix/combine.
Airing-Out, or letting a bottle Breathe, is an open bottle sitting for awhile out of sunlight to allow the nicotine to start oxidizing, and/or evaporate out some more volatile flavoring notes and/or alcohol.
I've got a few mixes where steeping helps. Only one mix where letting it breathe helps.
I don't do an RY4, but I understand it's one that usually gets some help from steeping. (If you like the bitterness from oxidizing nicotine, then letting it breathe may be a good idea too.)
No ingredient changed the game for me. It was finding the right flavoring level. I've found some that if they are 0.4% off go from great to ugh. I've also found some that can vary 5% without much of an issue. Flavoring level was such a big deal, it was my fist blog on mixing.