First since this is a safety device I'd look into filing a complaint with the NHTSA.
I'd then speak with dealer about this and ask why this shouldn't be covered by some good will from Porsche. At some point in the discussion I'd casually let it drop that I had been in contact with the NHTSA about filing a report. It is not like air bags are wear items, or you abused the car, or used it in a way that was inappropriate. No way should normal usage of a car compromise something like the air bag system.
If this doesn't help I'd still file a complaint with the NHTSA. It might tell you to get stuffed, but you never know.
Next price the parts that need to be replaced. Maybe you could buy the part(s) and do the labor yourself? 'course, I suspect a good percentage of that $4000 quote is parts cost, as I can't think it would require that much labor to replace what I suspect is the sensor that detects if the seat is occupied or empty.