They can't "retro-actively" push your start time after you duty on at 0400. The grey area here is what constitutes notification...that's in your contract, not really in the FARs. Typically a VM or other message does not count unless they talk to you in person or you self-notify BEFORE you duty on..

If you don't want that change today, tell them you were not officially notified prior to duty-on, so your duty-on time is legally fixed at 0400 and cannot be "undone".

From a FAR legality perspective, this on of those "if a tree falls in the woods and nobody is there to hear it, does it make a noise?" situations. Basically if you WANT to duty on at 0900 instead of 0400, nobody but you can say whether that happened or not.