I emailed it to a big box store to be printed. It was $25 more at both places I price-checked for 5x7s, so I ordered 4x6s without thinking it through. I'll keep my fingers crossed!
Because they're slightly different in proportion, if they merely shrunk the jpeg to 4x6, there will be some distortion. If they shrunk and cropped, you'll be fine. Good luck!
Ditto vc, they will usually crop rather than just size it down because it will be inevitable that you will come back to them asking why it looks so stupid. Hopefully it doesn't crop too much!