Combine 4 photos to make one image

I am assuming you want to print this when you're done?

Create a new document in the required output size and resolution by going to File > New.  See screenshot.  Hit OK.

Open your 1st image.  Use keyboard shortcuts Ctrl+A to select all.  Ctrl + C to copy.

Switch to the new document and hit Ctrl + V to paste first image onto your canvas.  Repeat with 2nd, 3rd & 4th images until you have a working composite with each image on it's own layer.   See screenshot.

With the direct selection tool selected, click on a layer to select it and move to desired location.  Use Ctrl+T to transform the layer's size, etc....

The key thing with Photoshop is to practice, practice, practice.  Everything gets easier the more you do it.  If you make a mistake,  use Ctrl + Z to undo what you did.

Do you have each version of the picture on a separate layer?  If yes than JJMack above is right. Here is one way to do that. You would go to the IMAGE tab (look on the very top right hand side of the Ps window).  Click on the IMAGE tab and go down to CANVAS SIZE.  The Canvas Size window will open.  At the top you will see the current size.  In the NEW SIZE section, increase the size of the canvas to accommodate all four images being placed the way you want them.  So if you have a 2" x 2" current size and you want all the images right next to each other in a line you would make the new size 2" x 8".  then arrange you images in the way you want.  Next look to the top right and find the LAYERS tab.  Click it and go down until you see FLATTEN IMAGE at the bottom.  Click Flatten Image to get all the images on the same layer and save your file.

Hope that helps.

If you're looking forward to buttoning up all pics together. Try following up this method (MAY HELP YOU): Get the photos in a single layer of a file > EDIT > Auto-align layers or Auto-blend layers (works well buttoning up stuff) > PLAY WITH THE OPTIONS

The moment you come up with the result, do let me know

Cheers,

Daniel

That should be easy. You have your 4 images layers use canvas sized and  increase the canvas by 100% width and  height and the arrange the layer the way you want them.