Quad Darts is the first upgrade on the first path for the Heli Pilot. In BTD5 Mobile, it costs on $510 on Easy, $600 on Medium, $650 on Hard and $720 on Impoppable. It allows the Heli Pilot to shoot 4 darts every volley instead of 2 darts per volley. In BTD6, it costs $680 in Easy, $800 in Medium, $865 in Hard, and $960 in Impoppable.
Generally, Quad Darts is used as an early-game upgrade when using the Base Heli Pilot. Similarly to the lower tier, the Base Heli Pilot finds most use on long tracks and maps where the infinite range is appreciated.
For MOAB Shove, it may be better to not buy Quad Darts for additional stalling but for Comanche Defense and Comanche Commander, this crosspath will be the best as they are DPS oriented and can obtain camo detection from external means.