The tanks are normal tanks. Big gun, lot of armour, very fast, tracks, etc. It can't just be a dedicated drone operator. (It can be part of the role but not the whole thing) You can add whatever you want onto the tank as long as its within the realm of the semi-sane. The technology level is near future. Being hacked mid combat isn't a concern. The primary scenario is conventional warfare between relatively equal powers. Although counter insurgency is somewhat relevant. Combat resembles that of today some-what closely. Without a loader what could a 4th crewmember do?
With a traditional turret layout:
Engine in the front or rear, driver in the hull, three soldiers in the turret. One is the (main) gunner, one the the commander, what can the third turret position do?
- Load the autoloader.
Typically an autoloader has only a limited number of ready rounds, with more stowed anywhere they might fit. The gunner could do that, but the gunner typically fills other jobs as well.
- Help maintain situational awareness. The driver worries where the tank is going. The commander worries what it should do. The gunner aims the gun. That leaves the ammo handler (see above) to worry about the sides and rear.
- Unit commander or staff.
In a traditional tank platoon, the commander of one tank wears two hats as the tank commander and platoon commander. So give the tanks with the platoon leader and platoon sergeant a more junior sergeant as dedicated vehicle commander, and let the platoon leadership lead the platoon.
Tanks without the platoon leadership could host an artillery observer or company or battalion staff, but that could lead to tricky command relationships -- what if an artillery lieutenant habitually sits in the senior seat of a tank commanded by an armor sergeant? Who is in charge?
With a futuristic tank layout:
Engine in front, unmanned turret with an autoloader, four crew in a protective capsule in the hull. With drive-by-wire controls, in theory everybody can do everything, but the training and the consoles are somewhat optimized.
- Two of the crew positions are optimized for weapons and sensor operation. The consoles are mirrored, but one is the commander and one is the gunner.
- Two of the crew positions are optimited for movement and sensor operations. Again the consoles are mirrored, but one faces forward and one faces rearward to avoid disorientation when the tank drives rearward at high speed. The skill sets of the driver and rear driver are interchangeable, and whoever is not controlling the vehicle handles the sensors.
Radio operator and IT guy.
Radio operator has been a historical position on tanks. While it makes sense to let the commander operate it the commander is also a busy man. Having someone else who can take note of orders coming in during the heat of battle or transmit information to unload the commander is a good idea.
But tanks are becoming more and more electronic, having someone who can manage the systems and solve problems can become vital, especially with how software can be altered easily on the field. This can be likened to an automatic transmission versus a manual transmission as well. An automatic transmission does not always switch properly for the situation at hand as it is programmed for a more linear acceleration which can make it less efficient, your IT guy can alter the software parameters to set templates based on the current goals and situation of the tank.
Your gunner still thanks you to this day for solving the "404, ammo not found" bug on the spot.
Not be there in the first place!
The interior space of an armoured vehicle is a trade-off between carrying engines, fuel, weapons, ammunition, and crew. The more you want of one, the less you have of the others. If you have one less crew member, you have roughly a 5ft x 3ft x 2ft volume of space which can be used for something else.
For example, an M1 Abrams has a fuel tank of 1900 litres, giving 265 miles range. That amount of space is an extra 810 litres, which would give you another 112 miles of range. Worth having, right?
- Manage communications during an engagement, e.g. coordination with other units or calling in air and artillery support
- Operate additional sensors mounted on the tank or reconnaissance drones to improve tactical awareness
- Operate defensive measures (e.g. smoke, active defenses) or a light secondary weapon for protection against infantry or helicopters.
Modern tanks have a lot of defense systems. Flares to spoof heath seeking missiles. Electronic countermeasures. Remotely fired machine guns. So one more soldier taking control of all these weapons while the gunner is mainly concerned with the main weapon would make sense.
Autoloaders are used specifically to reduce the number of crewmen. If you want 4 crew members, just drop the autoloader and get yourself much more efficient, and safe to use tank, as autoloaders generally are massive safety hazard in the tanks that use them.
That's why almost western designs from Cold War era pretty much exclusively had human loader allowing for much safer ammo storage. On the other hand, Soviet autoloaded tanks tend to burnout on any penetrating hit.