There should only be two wires on each injector, one hot wire and one that seeks ground through the PCM. The hot wire should be the same color to all injectors, but the ground seeking wire should be a different color for each injector.
With power off, unplug the harness from the injector and check the resistance of the pin on the harness (not on the injector) that the ground seeking wire is attached to and a nearby ground. Check a couple of plugs. There should be no continuity. If there is a low resistance on the wire for the #4 injector only, then the driver transistor in the PCM or the wire to it has shorted out. If it is low or high on all the ground seeking wires, then test again but with power on, but before you conduct this test, do a voltage check on each ground seeking wire to make sure there is no voltage on the wire, this is to protect the meter.
If you get a voltage on on wire only, the the issue is still with the PCM. If no voltage present, all wires should read high resistance to ground. If the #4 only reads a low resistance to ground, the problem is still in the PCM, but the issue may be something other than the power transistor.
If the #4 only reads a low resistance on the first test, unplug the PCM and retest. If it is still low, then the problem is with the wire and not the PCM.