Multiple Videos on One Screen

Can't help with Sony Vegas, but it's a fairly standard technique. Arrange each clip in the composite on a separate layer on the timeline, one on top of another. Then, for a four-way split screen, reduce the size of each video by 50% and adjust their position on-screen. You can use keyframing to have one of the videos to take over the full screen.

Track Motion and keyframes are the tools you want to use for this.

Remember that the top track has priority so place your central video on the top track.

There's a short video (4 min.) on the basics of Track Motion on the Sony training page at http://www.sonycreativesoftware.com/training/vegaspro

Mike