I have 4 videos which i want to combine it into 1 video file. Drag and drop should do the combining but i don't know how to crop it.
I am new in video editing, so any suggestion or even keyword will help.
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.
You need to mask each video. Masking is a way of revealing or hiding parts of a layer based on a shape that you draw. So in your case, you would draw a rectangle on top of the video layer you wanted to crop, and this would reveal only the area inside the rectangle. See this tutorial for more:
Having said this, Premeire can also achieve the same effect. After effects might be overkill for something like this.