Try breaking it down like this:
self.button4.hidden = NO;
self.button3.hidden = self.button4.hidden; // We just set self.button4.hidden to NO, so this passes that value to self.button3.hidden
self.button2.hidden = self.button3.hidden;
self.button1.hidden = self.button2.hidden;
It's just setting them all to NO
in a clever way