It becomes a unit with multiple save values. As such, depending on the position of the Shield Drone, the unit may or may not benefit from the 4+ Inv save.
For example;
Say you have a Drone (D) and two Fire Warriors (FW) being shot at by an AP4 Heavy Bolter armed Space Marine (HB) like so:
(HB) -----> (D)(FW)(FW)
In this case, your Drone has to die first, as it's in the way. You roll one 4+ Inv save at a time until either the Drone makes all the saves, or the Drone dies. If the Drone dies, any unsaved wounds will start carrying on into the rest of the Fire Warrior unit, but with only a 4+ save, their armour will be ignored.
However, if the squad was positioned like this:
(D)(FW)(FW)<------(HB)
Then in this case, your Fire Warriors die first, as the AP of the Heavy Bolter ignores their 4+ save. If there's still unsaved wounds in the pool by the time it hits the drone, you'll then start making 4+ Inv saves until all remaining wounds are saved, or until it is destroyed.
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