How turn off passenger air bag on audia4?

Unless your A4 was factory fitted with a switch or key (probably located either in the glove box or on the left hand side of the radio) to disable the passenger airbag, in which case it is very easy, disabling your airbag is a very expensive process...

The only way to safely disable it is is by having a disabling switch fitted retrospectively. This will cost about £970.00 at an Audi garage. You can however put a child in a forward facing car seat in the front, when the air bag is in use. However NEVER put a rear facing car seat in the front seat of the car when the air bag is connected.