If you really want to think about a fourth class (that isn't Psionics), you need to think about what the three original classes bring.
So the Mercenary is the Fighter, the Tech Specialist is the Magic User and the Smuggler is the Thief. These aren't concrete comparisons, they do cover enough bases to build a foundation for a fourth class. We'd want something of a hybrid class, one that takes elements of two classes and brings something new to the table, for example in Fantasy, the Paladin is essentially a hybrid class of the Fighter and Magic User, the Ranger is the Fighter and Thief and so on.
So what would that be in TiTS?
Mercenary + Tech = A combat character with a high level of technical know-how, possibly a frontline type character who uses their tech skills to grant them temporary buffs like Boosting Shields for a couple of rounds. Probably wouldn't have the damage output a specced Mercenary does, but definitely has more survivability. Possibly the Combat Engineer archetype or something to that effect.
Tech + Smuggler = A difficult proposition, combining some criminal tendencies with Tech skills. Not the tankiest character, but probably able to last a while if they need to. Or considering the TiTS universe, a role far more suited to Corporate Espionage Agent, slip in, get the info and get out.
Mercenary + Smuggler = A character that would have that much HP or shield strength. If they're forced to fight, they'd have to deal a high amount of damage. The character could be a sniper of some kind or far more likely, a dedicated Duellist.
Of course, this is just all theory and supposition at 4 in the morning, so make of it what you will.
If that’s the way we’re looking at it, look at Mass Effect. Solid combat and classing. Adept : Mage :: Engineer : Thief :: Soldier : Fighter, no?
With that logic behind us,
Adept : Mage : Tech
Engineer : Thief : Smuggler
Soldier : Fighter : Mercenary.
With that piece also behind us,
Vanguard : Tech + Mercenary
Infiltrator : Smuggler + Mercenary
Sentinel : Tech + Smuggler
Tech + Mercenary, I imagine, would be melee based, using a physical weapon for close combat while relying on Tech abilities for long range.
Smuggler + Tech, on the other hand, would focus on debilitating the enemy before dealing a single, ravaging blow, wether through Lust or Physical damage. Seeing as how this is the TiTS counterpart to ME’s Infiltrator, I’m thinking Kinetic Ranged focus, since Tech Specialist already uses energy. Maybe the Merc + Tech combo would focus on energy melee, too. I see Emmy’s Vampire Sword being essential to such a build.
The Sentinel part is where this thread’s Zoologist class would come into play. The class would have no attack bonuses, not even the iconic Double Attacks all the current classes have now. It would, however, have incredible boosting and sabatoging power, and would most likely rely on its companion for damage, wether Siegwulfe, Varmint, or Tam-Bot. I imagine it would play a lot like the Witch Doctors of Diablo. In the same vein, the Sentinel’s parasites would also attack autonomously, perhaps by an acidic/pheremonal spray from Mimbranes, or the Cunttail/Cockvine lashing out at potential sneak attackers.