So, you are driving six injectors (for a V6?) from one single ecu channel? And the channel has the capacity to drive 6 high impedance (12Ohm) injectors, so a total of 2 Ohm load and 6A of current?
If so, then batch the 6 4Ohm injectors (this makes 0.67Ohm) and put one 1.3Ohm 50W resistor in series of the batch.
Just make sure your ecu is up to this.