I think the better question might be, "How mad will you be if Mass Effect 4 doesn't retcon the ending of ME3?"
Simply put, it will someday be necessary to resolve that ending and pick one to be "canon." Red, blue, or more likely a purple amalgam (but obviously never green) becomes "what happened at the end of the Reaper War" and the galaxy moves forward.
Many people seem to be jumping on the Andromeda bandwagon, and the image of the concept of what might be a galaxy map is adding fuel to those flames. As I see it, taking the series to another galaxy is almost as craven as making a prequel, because that would literally be running as far away from that hot mess of an ME3 ending as they can possibly go. How pitiful is it to think that BioWare broke Earth's galaxy in the MEU so badly that they are unable to fix it?
I hope that they're just gonna bite the bullet, face their narrative mess, and start cleaning it up. I think that there is almost no-one who would actually and sincerely argue in favor of preserving the uncertainty and silliness of the ME3 ending intact going forward. Casey says that they've listened to feedback that overwhelming says that we want to move forward and explore new regions of space, but I hope he understands that we want to move forward with the Mass Effect galaxy that we already know, and explore new worlds while still being able to visit the Citadel.
Maybe Casey and Mac are done with the characters and places introduced in the trilogy, but I'm not. I'd be willing to bet most of you aren't either.