I have a 2003 Mercury Grand Marquis. The air bag light flashes a code of 3, pause, 4, pause, repeat, repeat, repeat. I have not changed anything. I did go change the battery after it started flashing. Still same thing.

That code relates to the Passenger Pretensioner circuit:
Restraint System - Safety Belt Pretensioner Status (Passenger Pretensioner Circuit Fault)
Check underneath and around the passenger seat to see if a connector has gotten unplugged. You should be looking for a Yellow connector. If you don't see anything obvious, you may need to contact a technician or shop that can diagnose that system to find what has failed.