Where to find Lester since Chapter 4 in Gothic 1
Since the fourth chapter of the game, Lester won't be at the Swamp Camp anymore. You need to talk to him when Diego gives you the quest to reunite the group (Diego, Milten, Gorn, Lester) following the death of the Magicians of Fire.
Lester is in the fortress where you collected the Focus Stone in the third chapter.
Head towards the Swamp Camp, after the first bridge of the Goblins on the right, and before the second bridge that leads to the Orc Cemetery, turn right and climb the slope until you reach a narrow passage that brings to an open area populated by hounds. You should already have cleared the place when you reached the Focus Stone.
Cross the bridge on the left, guarded by a stone Golem, and go upwards until you reach the fortress. Here, at the library on the floor level, you will find Lester reading a book.
Also read the complete Gothic 1 guide.