hi,
Thats one of the problems with the standard BCD to 7 seg decoders, the 10,,15 are blanked!.
If you want to display hex numbers 0 thru F [0000 > 1111] you would need a BIN to 7 seg decoder, that would display 0 to 9 and A to F.
I have not seen that type of ic for many years.
You will require more decoding of the binary output SUM for conversion into BCD before you connect the 7447's.
The simple way would be to have a 220R on each 'S' output pin on the adders and drive a single type LED,
the result could be then read in binary.
I will look thru other options and let you know.
Other members, I am sure will also have some ideas.
If you understand let us know.
EDIT: Have a look at this site.
**broken link removed**