Found it!

With the help of Kylia on the McShepSlash yahoo group, we now have the SGA/Die Hard 4 fusion fic:

Live Free or Die: The SGA Fusion

Author: sian1359

Pairing: McShep
Category: Action/Adventure; H/C; AU
Rating: Adults recommended because of violence and language
Word Count: ~44,000

Disclaimer: Characters and Storyline -- Not Mine

Spoilers: Severe for the LFDH movie and character deaths that have taken place during Seasons 1-4 of SGA