I'm assuming you are referring to photos that are being placed in Illustrator (as you refer to measurements in pixels?) In that case for print the images would need have a 300ppi resolution, which I'm thinking based on the 1200px and 4-in reference you give, they already are. (I checked that by creating a new photoshop document 4x4 inches at 300ppi resolution, and then check the amount of pixels choosing Image > Image Size).
Here's what you can do:
- Create a new A3 artboard in Illustrator
- Then place the images int to the document by choosing File > Place, navigate to the image, then with the loaded graphics icon click on the page to insert the image in the document (do not click-and-drag). You can position the image on the page afterwards by selecting and dragging it with the Selection tool. Using this technique retains the image size and resolution.
// note: you can select multiple images at once and then place them one by one, if they need to be placed in a particular order, press the arrow keys on the keyboard to toggle through the image that are loaded in the cursor.
You could probably fit 8 images on an A3 (2 columns wide, 4 rows deep).
If you resize them in Illustrator, for example make them larger, the effective resolution of the image changes. Think of it as making the pixels larger, and thus you'll end up with less pixels on an inch = reduction in resolution. With the image selected in Illustrator the Control panel will display the current PPI amount, so you can keep an eye on the resolution.
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