It’s not like a 4x4 truck, as in a F-series truck, where you have a transfer case to engage manually or the operator engages as they desire and then it’s always on regardless of speed.
For vehicles like a Rav4, CRV, Element, etc, the 4WD/AWD capability engages automatically based on wheel-spin and then turns itself off. We have a Rav4 and if we had the Limited trim we could press a button to manually turn it on versus letting the car determine it needed 4WD based on wheel spin.