I did this in 3rd edition, created a small mailbox-sized crawler drone with a weapons mount capable of taking up to LMG class weapons. Really no other upgrades as there wasn't any more room.
They were slow as hell but it didn't matter, cos their role was to be portable gunpods that could be stuck to anything and provide support fire. Stick em on top of a vehicle, on the ceiling, even on the Troll's shoulderpad. The crawler legs were capable of moving around a little but they were mostly there to grip things and adjust the position of the weapon.
Probably wouldn't be too hard to stat up in 4E.
-k