Stitch PDFs with a small overlay
Whilst I am unaware of any easy to use software dedicated to stitching smaller images together (I'm sure there are some tool available on GitHub to do this); it is possible to do what you require simply using the stock Preview app, following the steps below:


Firstly, ensure your A3 images are saved as images rather than PDFs. Then, open the first A3 image in Preview.
Select all (⌘ + A), then copy (⌘ + C)
Select "Tools" From the menu bar, then select "Adjust Size..."

Firstly, click the small padlock icon to make sure the size adjustments are not locked. Now, assuming you are using landscape images, enter : Width = 9933 Height = 7016 PIXELS (ISO A1 paper size). If you are using portrait; simply swap the values.





This will instantly stretch the first image to A1 size. Now paste in the original first image (⌘ + V) and position it to the top left hand corner, ensuring to use the arrow keys for fine adjustments.
Now, open the second image in a new Preview window, select all (⌘ + A), then copy (⌘ + C) and and paste it into the new image we are creating (⌘ + V) and move it to one of the corners, ensuring to use the arrow keys for fine adjustments. 
Repeat the step above for the remaining two pictures until you are satisfied with the positioning.
Finally, save as a new image.


Hopefully, you should now have an A1 sized image made up of your 4 component A3 images. Of course, Preview is in no way regarded as a professional image editing application and has only very limited functionality. Therefore, if you are willing to invest some money (or already have), Adobe Photoshop makes virtually the same process a whole lot easier. Alternatively, you could use the GIMP package for a more dedicated and less costly image editor