Four days apart isn't that much. Turning the eggs is most important early on in the incubation process, and not so important nearer the end. I think you could compromise a bit here. If I were you, I would stop turning the first lot of eggs on day 18, but wait until day 19 to raise the humidity and go into lockdown properly. That means that the second lot of eggs will be getting turned up until their day 15, which is probably good enough. And after you go into lockdown, just keep your humidity high until the second lot (hopefully) are done hatching.
I've done quite a few staggered hatches all in the one bator and they'rve worked out okay. I do get a decreasing hatch rate with the second and third batches of chicks, probably due to unsuitable humidity and bacteria in the bator after the first and then second batch has hatched, but that's with eggs due 14+ days apart. Just four days between them shouldn't make much difference. Good luck!
