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Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the difference between AWD and 4x4?

I recently saw a post about a truck that said that the owner converted their truck from AWD to 4x4. I'm wondering how the two are different because they both follow the principal of all four tires moving. Does one work mechanically different from another? Does one provide more power? How do they work?
gpt
AWD has a fluid coupler between the front and rear wheels so it allows for some wheel speed discrepancy between the front and rear wheels.
Therefore AWD can be used on dry pavement and make tight turns without cause damage to the driveline or chewing up the tires.
AWD is meant to be used in any and all conditions, all the time.

4x4 is selectable between 2 wheel drive and 4x4.  When you're in 4x4 the front and rear drive shafts are "locked" together - turn at the same speed.
Therefore it's not safe to be used on dry pavement and making tight turns (like in a parking lot).
4x4 is meant to be used only in "low traction conditions".

ELI5: 4x4 makes front and rear wheels turn at exactly the same speed - not good on dry pavement.
AWD allows for small differences in speed of wheels so you can use it all the time (like making tight turns, and parking).