There already exists software for such task, such as PosteRazor and such.
However they all have one huge flaw:
They require using a Scissor and have sometimes inconsistencies applying the glue line
if a image like this:

could be turned into multiple A4's with glue line like this:

(orange is the glue line
i'd be totally satisifed, however I think short of imgmagick I cannot imagine this to work.
So the rundown is:
- Take any image
- cut it to multiple A4 images while adding ~2cm of empty glue line like pictured above
(Optimal would be if it took every image given to it and automagically cut and slice it to the perfect amount of required A4's.
(In order to keep 100% image size)