It is. And it's easy. Might seem like a lot, but I just explained it in details
Open up all your images in photoshop one by one, by chosing file>open (or Ctrl+O) and then locate your file (you can open more than one image at a time by highlighting more image, this is done by holding down shift or clicking the files while holding down ctrl).
The last image you loaded should be visible on your screen right now.
Now make a new file by chosing file>new (Ctrl+N) and set the canvas size to your desired size (this can be altered later if you find it to small or to big). This will create the empty canvas, where we are going to merge the 4 images.
Now, on to adding the images
On your file/naviagation tray (@ the top) you'll find something called "Window". When you press this a dropdown-menu will apear, at the bottom of this menu you'll see the current files/images you have opened in photoshop. Now just go ahead and click the first name (this will change window from your empty canvas to the first image you loaded). Then hit ctrl + a (marque all) and then ctrl + c (copy selected). Now go back to your empty canvas (again, in "window") and hit ctrl + v (paste copied), this will paste your first image onto the empty canvas. You can now resize your image if you need to by going edit>free transform (ctrl + t) (holding down shift while using this function will keep the image in proportions).
Now go ahead and select/copy/paste/transform the next image and so forth. If you need to change the size of the canvas where you add all the images just go image>canvas size (alt+ctrl+c) (if it's to big, I'd go ahead and use the crop tool on your tools palette (shortcut: "C"))