I can see y. When in 4wd, are the front and rear axles actually locked together? On some all wheel drive cars they are Not. So maybe being in 4wd just hooks the front axle to the engine and not directly to the rear axle.
It is easy to see a question? If the front and rear axles are locked, that means there is no differential between them. Really? Could be, but easy to see such confusion?
You are right, it's easy to imagine the question given the variety of incarnations of multiple wheel drive!
Cars are a weird reference. There are so many different schemes. I think "all wheel drive" most often refers to having a partial degree of coupling between front and rear, often a viscous connection, so that if you're going uphill in the snow and your primary drive wheels start to spin, the other wheels start to contribute too. Most reasonable to think of this as transferring more and more power to the other wheels, in proportion to how different the front and rear wheel speeds are. It makes life easier for the drive train because when traction is good and all the wheels turn at the same speed, there is no torque being transmitted to the non-primary drive wheels.
Cars are also a less useful reference here because they always have braking on all the wheels. Beyond that, more modern designs monitor wheel rotation with a microprocessor and fire the brakes on individual wheels to remedy various modes of traction loss or control loss. None of which applies to tractors!
And there could be a differential between front and rear, but that's not the only way of transmitting power. In older 4WD designs I think they're rigidly locked together, which does let torque build up (according to variations in wheel speed and the tendency of the wheels to slip a little bit).
AFAIK tractors rigidly lock front and rear together. That can build up torque, and rip up sensitive surfaces especially when turning.
I saw a crowd of young men pushing a small tractor through the woods after the main gear shift lever broke off. Fortunately it was in neutral. But, when they started turning around a clump of trees, suddenly the effort was Herculean and slow. I noticed it was in 4WD and suggested they take it out -- suddenly they were making easy headway again!