After Effect CC - How to combine and crop videos
After Effects and masking are both total overkill for this problem.

First you want a video editor, not a complex VFX system, as your baseline.  If you have After Effects, you likely also have Premiere Pro.  Drag and drop each video onto a separate video layer.  In Premiere Pro, each video layer has an Effects tab that includes a Motion effect.  The Motion effect includes scaling and translating video (shifting it left/right or up/down).  Scale each video to 50% (they should all scale toward the middle of the screen), then translate each video to their respective corner.

The last piece of the puzzle is to crop the videos as you wish.  In Premiere Pro, there's a video effect in the Transitions category called Crop.  By cropping left, right, top, and bottom by a certain number of pixels, you will get the rectangle (or square) you want.

But almost any video editor at all has Layers, Scale, Translate, and Crop.  That's what you need to solve your problem.