Click on the crop tool and then enter the dimensions you want in the empty boxes at the top of the window. You can also enter the pixels/inch here in the third box if you wish.
The crop tool will now crop to these dimensions. Open one photo up and then enlarge the canvas to A4 size (see below) You can then drag the other photos on to this page and you're away.
You should be aware that your printer may not print to the full A4 dimensionsd and you might realistically be looking at around 200x270mm. Consider too that you might want to leave a small gap between photos to be able to cut them up.