Dark Forces = New engine from scratch (although some rumor-mongers allege that LucasArts "hacked the Doom engine looking for ideas" and this explains why they never released the source (rather than for their well known stringent liscensing practices I guess). The engine was called "Jedi."
Jedi Knight = Same design team (basically) as DF1. New engine from scratch ("Sith").
Mysteries of the Sith = Modified Sith engine, some people that worked on JK returned, but otherwise same design team as "Outlaws" the only non-SW FPS made by LucasArts.
Jedi Outcast = Raven, using modified Q3:TA engine.
Jedi Academy = Raven again, using further modified engine as used in JO.
JK4 = nobody knows who will make it (if they make it) and using what engine, we can only guess. ; )
A lot of us are guessing if Raven DOES do it they'll do it using Doom3. But again, that's pure speculation.