Pg. 21 of the Zverograd campaign book states that each bomb can target one unit. Two bombs can be used to resolve the bomb as if it was a sustained attack action (note: bombs are not an attack action, they result from move actions.)

The SSU MIL MI-47D "Death Rain" has 2 bomb weapon lines with 2 bombs each.

The question is: Can this Aircraft drop it's entire payload on one movement action? Two bombs on two targets or one bomb on upto 4 legal targets?

Yep. As long as you have range in your rear arc, you bomb away at up to 4 units.