You need to use Track Motion to resize the four video tracks and place them in the proper corner. If you want "quick and easy" - check out the Video Wall tool in Tsunami.
Do a search for "PIP" or picture-in-picture. As Ed says, it is usually done by putting each video on a separate track, and then using track motion to position each video. You sometimes also use the cookie cutter fX.
Easiest way I know is to put all your videos on 4 tracks and using 'Pan&Crop', shrink each video down and place whereever you want on screen. They may not fit perfectly, but you can use a Backdrop(on the 5th video track, which should be on the bottom.) and make 4 screens fit on there very easily this way.