If she doesn't nap I would do 5.30/6pm. She will be OT but she's going to be regardless if she doesn't nap. I would think 4pm BT would get you into a very messy place indeed with EWs.....
For nap if she falls asleep by 3 I would maybe let her have an hour, get her up and then do 6.30pm BT (ie your early 'set' bedtime).
There's no right answer though on days like these (we've had a few
) so if your mummy instinct tells you something differently you know her far better than we do
If you're giving set naps a fair go though you would need to keep sticking with the times tomorrow. (((Hugs))) xx