How does the 4 wheel drive work on a 2006 Jeep Grand Cherokee Laredo?

I recently bought a used 2006 Jeep Grand Cherokee Laredo 4x4, no owners manual. The sales person told me he thought it was "automatic" 4 wheel drive. It is not an AWD, it says "4x4" on the vehicle. There is no on on the inside to turn it on. Expecting snow tomorrow.....