C.B.
I really like your preschool mom idea. If there are stay at home moms, then maybe they'd be willing to make a little extra money every now and then. Plus, your 4 year old will probably think it's a drop off play date since she'd be going to play with one of her peers. This might make the separation a little easier for her. Another thought is to go online to sittercity.com or care.com. You can post your situation, and you might catch some local college kids / nanny's / part time workers who would be willing to be backup in this situation. If you did the one month trial & collected a few names, hopefully one of them would be available in a crunch.
Personally I would probably avoid hiring someone in a day per week unless you're trying to phase out your nanny.