Double incubator effectiveness

Don’t assume that yet.

Yes, you should assume that. Don't start your eggs yet, it's always been only for eggs that began incubating during the window.

It is not the eggs dropped during this window that cause the incubator to be more effective, but that they increase the effectiveness of the incubator itself. Therefore, wait until the window starts to incubate your 4 10k eggs and you should still see the benefits of increased incubation effectiveness.

If it's like every other event it will only be for eggs started within the window.
But if you want the eggs hatched why not just start them now and they'll probably be hatched before the event even starts. Not like there's any bonus related to egg hatching now or then.