battery

It could be the battery itself. A battery over four years old can deteriorate from small internal shorts. You may also have some parasitic draw from a bad circuit including a "leaky" diode in the alternator. A hydrometer (electrolyte) test of the battery can show that weak cell. That's where I would look first.

Couldn’t have said it better myself. Listen to hornet_2497 because those are the only possibilities. If you start it and let run a few minutes each day and as a result each and every day you are again able to start the car and let run a few minutes allowing for this process to continue repeating then you clearly have a working alternator. Otherwise even with a new battery, in time you’d eventually have a dead car and could not start, run then start run repeat day after day. But a faulty battery would indeed create this situation. As well something somewhere could be continuously using your power also creating this situation. In that case check for things like your trunk lights remaining on after you have closed it. Or something I have had happen that is annoying. A very small leak in one of your air suspension air bags. You won’t necessarily hear the air escaping but maybe for example after two hours of sitting the air bag in question has lost enough air that suddenly the compressor kicks on automatically to refill it because the sensors are now unlevel. And this process will continue to repeat endlessly and as a result draining your battery. Hope that helps. Good luck.