Worth promoting?

wait for +Atk -Spd, then promote.
the +1 4* will mean nothing anyways once you promote but if you used skill inheritance i would still merge them to keep the skills
unless you are desperate for a blue armored unit, there is no reason not to wait (especially when summer corrin and summer robin will be coming back "soon")

Well, in hindsight, merging her as a 4* probably wasn’t the best move. But in all seriousness, I’d say promote her, and grind away. Then when you get a better IV (+ATK-SPD) merge them and you’ll have an optimal IV with a merge in place.