How do you tow a 4x4 vehicle?

Depends on a number of things. If it's a 4x4 with a manual transmission and/or manual transfer case (or a four button electronic transfer case with a neutral button), you can put the transmission and transfer case in neutral and tow it how you please - so long as it's a 4x4 with a longitudinal engine setup.

Some vehicles, such as the Ford Escape, use a transverse engine setup and have a transaxle (much the same as any front wheel drive vehicle does, with the addition of a transfer case). Flat towing such vehicles is never recommended. For the front wheels, you would either have to dolly them or install manual locking hubs and keep them in the unlocked position. For the rear, since these transfer cases typically don't have a neutral position, you'd have to disconnect the driveshaft if those wheels were going to remain on the road. The simplest way to do these is to trailer them.

On vehicles with automatic transmissions and transfer cases which do not have a neutral position available, you're looking at disconnecting driveshafts if you want to flat tow them. For the steer axle, you can unlock the hubs if you have manual locking hubs instead of disconnecting the front driveshaft, but the driveshaft to the rear axle will still need to be disconnected.