What happens if you leave your truck in 4x4 all the time?

Depends if it's all-time or part-time 4 wheel drive.
All-time 4WD is designed to be driven on dry pavement, mud, snow, sand, wherever..
Part-time 4WD is designed to be driven only on snow/sand/mud, loose dirt.
Part-time 4WD locks the front and back axles so all wheels will always turn Turn a (slow) tight circle in a parking lot and it will jump, bump, grind because the front, inside wheel cannot turn slower as it needs to.
Either way, your mileage will go down a bit.

youll use a hell of a lot more gas. and if you get an uneven wear in the tires or if you have diferent types of tires on the wheels, you will bind up your driveline and lock up your truck and ruin it. make sure all the tires are EXACTLY the same. even the same brand and tread. and check every month or so to make sure the tires aint wearing down unevenly.

You will ruin it. Read the owners manual, it is a good starting point.

You WON"T notice any difference in mileage. You will however tear up one of the rear ends.

You WON"T notice any difference in mileage. You will however tear up one of the rear ends.