IIRC the explanation in the technical manuals was that the 4 nacelled configuration allowed extended cruise times at higher warp allowing these ships to conduct longer range missions.
This was accomplished by operating one pair of nacelles at full power, while the other pair of nacelles was powered down or operated at low power levels. When the coils in the active/hi-power nacelles reached their peak operating time, they would be throttled down while the other pair would power-up allowing the ship to maintain it's maximum cruising speed. Alternating nacelle pairs would allow these ships to conduct longer range missions than any standard dual nacelle configured ships.
There also is a reference, in the DS9 technical Manual, to a Nova class starship outfitted with 4 nacelles that is used as a high speed courier vessel.