It could be a variety of things, as others have stated.
In my case, my alternator was going out and I would get the 4x4 light on cold mornings. The problem was its module along with others weren't getting the correct voltage because the alternator didn't kick in until the engine warmed up.
Best to get a scanner with the ability to see BMW trouble codes and live data from the modules. I got a Foxwell NT520 Pro.
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