If you're doing any Multi-Player maps for Quake 4 that have areas of 'terrain' in them, keep an eye on collision problems that may occur when you 'connect' those types of areas together with the flat brushwork of man made objects.
Because the collision system was simplified for MP it has some odd side-effects when brushwork (including clip brushes) is buried inside models; in situations where you want to create 'transitions' from areas like a man-made doorway to a mine shaft, collision problems can happen where the mine shaft model and doorway overlap and meet up - the doorway brushwork being constructed so it's partially buried in the model.
One way around this problem is to create clearly defined areas that don't overlap, thus avoiding the need to bury brushwork. Obviously these means a little bit of forethought and ample use of the 'export selection to OBJ' feature to allow you to work directly with map sections in your 3D application of choice; using the imported OBJ section you simply build around it to get precise alignments of mine shaft to doorway (for example).
