Hmmm, but that still leaves us with the part where the Strogg first arrived at Planet Earth, at that point they probably looked different because they still had to use some alien lifeform body and not our pretty shells, and they probably stacked up some ‘spare parts’, in case one of this bodies ‘malfunctioned’ 
So, wouldn’t it be a cool Lvl4 Strogg reward to beeing granted a respawn (once only) in an alien body with some special abilities (extra fast sprinting + vicious melee attacks, or stealth abilities, or…)? You could even make it borderline overpowered, as you have to level up high enough to get this super-body first, and after beeing killed have to respawn in a human body again… would be pretty cool I imagine :evil:
The GDF could get some other Lvl4 extra one-time-only reward, like an extra tough tank or such… well, probably already too late in the development of the game to implement and test such things, but maybe a modder will pick up the idea 