montage

I have four videos that I would like to tile in a 2x2 fashion to make a new video. Is there a way I can do this easily, preferably free and under Linux? I am willing to program a moderate amount, perhaps in order to interact with some library, but unwilling to write an entire video-processing program myself. You may assume that the input and output videos are in whatever commonly-occurring format is most convenient. An analogue of the gm montage command (for images) would be fantastic. I am currently using GStreamer for a similar project (lecture-capture) myself. You are probably looking for