Don't worry about it--no problem!

When your I-485 is being processed, small immigration mistakes will be forgiven( there is a law/memo or something like that.). And, the worst case sinario, you were employed on 4h, but you might not have started working till 11th, or getting paid till 11th. ---If you were asked. Don't worry yourself to death. You will be fine. Everybody has a little bit here and there. The IO looking at your file will never pay that much attention to your emplyment letter. Best luck.