you just convert the sr04 to i2c using this arduino adapter, just set a different i2c address for each adapter then you can run as many as you link without changing ardupilot.
there is this adapter if you want to run the sonar is serial mode then convert it to i2c, its a little more accurate as the timing is being done by the sonar its self.
you can also connect multiple sr04 into a mavlink sonar array, but i have not tested this on a vehicle.