If you self notified on your day off then you're start time will be whatever the change was. If you didn't, then you would start day 1 at 4a. If they call you at 4:01 to say your RSV period starts later in the day it's illegal. They'd have to give you at least 8 hours of rest to start you later since your duty period already started at 4a. Now, if that's legal per your contract to have 2 RSV duty periods in a 24 hour period you're good to go, which I'm pretty sure it's not.