I just looked at the code for A4 embedded to refresh my memory. Joseph shows you some instructions I initially wrote on how to customize a certificate in MoodleDocs many years ago. However, here is what I see looking at the mod>certificate>type>A 4 php file.
You should copy and rename one of the folders in Mod>Certificate>type.
On or about line 39 you are seeing some x,y settings for image placement. It does not matter what size paper is in the printer. You are going to figure out where the elements you want should be located on that page. For example the Seal x value is 230 and y value is 150. Notice that the pix>seals images are all about 80x80, if you use a seal you probably will resize the image to say 20x20. As I recall, the upper left corner of the image is going to be at the 230,150 mark reguardless of the size of the image.
Then you are going to resize the text. Around line 91 is the code. So line 93 gives the code to print the title. It starts at the x,y point and so forth.
I used to comment out the lines I did not want to appear. I also printed up a stock certificate, then wrote in where I wanted things to appear (like x,y would be 10,30) and what the image was and how big.
And if you are really clever you will plan out your page certificate pdf such that it has the "cut here" dotted lines, so your student can get a "twofer".
Hope this helps,