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That could be one of several problems. The easiest to fix would be, if you have left the microwave door open.
But it is more likely to be a simple malfunction of the control circuitry. This means, it may well need servicing, or replacing.
However, first tell me, how many beeps are sounding? if two beeps, that's the steam sensor signal... and, it may be falsely sensing, if that has kept on and on and on, sounding.
Also if you need advice as to where to get the microwave serviced, or how to have a house call come to fix it, let me know your local zip code, please.
Microwaves don't contain batteries.
Unplug it, just to see if that stops the beeping. If it does not, you have, humorously speaking, a monster microwave with its own internal independent power which is beeping ominously... and yes, taking it to a repair-service, seriously speaking, would be a good idea.
I have no data on such a one-beep ea. 4 min.s, it seems to be a recondite error condition.
If the beep persists when switched Off at the wall, but not when unplugged, then you have a power supply (perhaps) that is defective, inside the microwave.