Actually the A380 will only have reversers on 2 of its 4 powerplants. This is to save weight. I' believe on the the inboard engines are fitted with reversers.
Generally speaking reverse thrust only reduces the wear on the brakes by allowing the use of a lower braking force. The actual "legal" performance of the aircraft is calculated without the use of reverse thrust and purely based on what energy the wheel brakes can absorb. To summise without reverse thrust the aircraft is more than capable of stopping in the required distance.