I've thought of that. Medics can't heal themselves, and Techs can't repair their own armor (otherwise, it might unbalance things in single player), but Medics start with an adrenaline powerup (which fully heals them, once), and in deathmatch mode can use Strogg Health Stations (otherwise, they're useless in DM). Techs start with 25 armor, and can manipulate more things across maps than the other classes.
Techs in Quake 4 are always doing things that other classes don't have the expertise to, like Strauss trying to overload that Tetranode halfway into the game, or Newberry cutting the locks to that one door. An example of how we're using this, in one of the maps I'm working on, Techs will be able to use a console to turn the lights on in a few corridors, to make fighting a few stalkers much easier, will be able to shut down some turrets and even open up a small weapons cache (throughout the map and not from the same console). That capability alone makes them my favourite class

.
To make things more balanced for certain game modes, like Last Man Standing and Possession, things like doors that only Techs can open will be permanantly opened. Otherwise a Tech could walk into a room only he can open, and wait there for the rest of the game. Non-critical things like turning on lights (By the way, the lights in that map will be permanant in single player, but only be on for 30 seconds in deathmatch), or permanantly unlocking a door will still be Tech-only.
There will be a small indicator showing what a Tech-only can manipulate, and a few Tech-only things are in the demo map.
The health stations the Medics can use make for interesting gameplay, as well. They can heal a player 10 times in 10 health increments (much like the Doom 3 health stations), but recharge after a minute or so (at which point, they play a sound and go through an animation, indicating they're full again). Since it's so story-unfriendly, it's only for deathmatch modes.
In case you're wondering why anyone would play as a standard Soldier, the Soldier has an extra 50 health and starts with 3 flare powerups. Two Soldiers, a Medic and a Tech make for a really good team.
These classes really made the game a lot more fun, in multiplayer.