Definitely different schedules. I agree the 4 year old is done napping. She may need some quiet time with books or quiet activity, vs. actually sleeping. It's more important to keep the night-time schedule as is.
Have you seen the little foam chairs that fold out sort of like a "convertible" couch? Kids in our family used those - they each had their own chair, and it folded out to form a "bed" but it wasn't the same as their regular beds. They'd curl up in it and often fall asleep when they were resisting a nap. Maybe you could put the older one in that while the younger one slept?
You'll have to tell them they are doing quiet time separately from now on - don't tell the little one that the big one doesn't need a nap anymore or that the older one is getting "special time" with Mommy! Tell them quiet time is non-negotiable, and put in a simple clock with a "don't move until..." time.
What do you do now when one wakes up before the other? Does one wake up the sleeping child, or just leave the room quietly upon waking? If the older one can lie there for 30 minutes with a book without the younger one seeing her doing it, then she can tiptoe out when the clock says it's time. The younger one doesn't need to know the older one isn't sleeping.
This will keep you in the "quiet time" habit for when the 3 year old wants to give up naps but really isn't ready - there's always a 6 month period of torture when an overly tired child has stopped napping! Mine napped well past the age of 4 and he still got run down in the afternoon until he was about 5.