It's not normal. I'm interpretting a little based on experience, but the only time mine beeps like that on me is when it has to force shut down while a game or something else is running and won't close properly. Like for example, if you have a game freeze and you have to hold the power button down to get it to shut off.
I've seen this happen every once in a while like if I'm in a multiplayer game and it gets stuck searching for a match or something. The game becomes slow in responding, so if you go to the menu and force quit the game, it will do the four beep and restart the system.
Bottom line, if it is doing that only rarely, then don't worry about it. If it does that every time you turn off the PS3, you may have a problem that will get more serious in the near future.