Yes, you can apply for EAD anytime after your get your I485 receipt. Right now, it is okay for you to switch to H4, of course.
But if you are going to work using EAD, there is no need for you to change into H4, since it will be invalid after your use of EAD.
I think you are safe now, if your hu*****and's I140 is approved, and with a pending 485. You can stay legally and work with EAD until your GC arrives.