Just a thought, a legged 4x4 model will probably look pretty silly and will be a pain to model. Like a lot of games with the moon-walking sliding legs thing, going across the ground, I'm sure you've seen it. A tracked or wheeled robot drone is ok. Wheels and treads are still efficient and able compared to legs, even in the future. Treads can be covered with a skirt and easy to model.
Maybe the the alien tank is just a big sphere, it rolls around, opens up into a shooting platform when it stops and shoots when it needs to shoot. Or it just has two or three wheels, with a turret. Or maybe It's a just a wide, squat, meat-based humanoid cyborg life-form on two legs.
The game universe might not have 'antigravity' or anything, but maybe a VTOL drone with anti-matter fueled rockets keeping it level and hovering. Like an unmanned, green miniature version of the alien spacecraft, for mining asteroids or whatever, but it's just a recon drone with a plasma cutter on it, the aliens use it as a combat weapon out of desperation in the late game.
I get with sci-fi stuff, people try to think of something more interesting then a box with treads and a turret on it, but the walk'n mecha-legged thing is getting even more trite. A tank with legs and a turret is just awful.