No juice to injector 4

No, you cannot because it is being energized by the computer to inject the fuel at a specific time. If you tie into with the fourth either they will both not work or 3 and 4 will pulse at the same time.

Keep chasing the wire(s) back to find where you are losing the connection.

The 12 volt supply is already spliced to all four injectors. You'll find each one has a gray wire. If voltage is missing to just one of them, peel the harness apart to locate the corroded splice or cut wire.

That's different. All four injectors have a common 12 volt feed wire spliced together. The control wires are the other ones in each connector. Those are grounded individually by the Engine Computer when it wants to pulse one on.

The fastest thing you can do is switch two injectors, erase the fault code, drive it and see if a code sets for the cylinder you moved the suspect injector to.

That is not the way professionals would fix this. If you can get your hands on a "noid" light, that will show if the injector is being pulsed by the computer.

Originally you said you were missing the 12 volts to one injector. All four must have 12 volts on their gray wires when the engine is running. If it's missing on only one injector, there's only a few inches of wire to inspect and one splice. It's a lot easier to fix the damage than it is to solder in another wire.

If it's the ground / control side that isn't being grounded by the computer, all four injectors have their own wires, and there's four "driver" circuits in the computer. Some engines years ago used "batch-fire" where two, three or four injectors were fired at the same time, and there were typically two batches, or groups. That reduces the number of circuits in the computer but it guarantees some of the injectors will squirt their fuel when their intake valve is not open. With sequential-fire, each injector is timed to open at the right time.

My fear, besides incorrect timing, is each driver circuit is designed to handle the very high current of one injector. If you add a second one, that will double the current and possibly take out that power transistor. Chrysler used to run fresh intake air through their Engine Computers to cool them, and they had almost no failures. GM had a real lot of failures, although not necessarily for this problem. I'd feel pretty sad if you destroyed your computer.

Gotta sign off in ten minutes. I'll be back in two days to see how you're doing.

A noid light is a special light for testing if the computer is pulsing the injectors. You have to select the proper light for your engine, year, and plug shape. Auto parts stores should have them. The guys who visit shops each week with their tool trucks will have them too. Those are totally different than regular 12 volt test lights. Injectors are typically pulsed on only for about 35 milliseconds. That isn't long enough to get the bulb in a test light to glow.

You CAN use a regular test light for the 12 volt supply to each injector. That's the gray wires. As I mentioned before, all four injectors' gray wires are spliced together. If you're missing 12 volts to just one injector, there's a break in that wire only as far as the splice which isn't going to be more than a few inches away. Be sure the test light's probe is making good contact with the terminal in the plug.

Yup. The Engine Computer uses signals from the crankshaft position sensor and / or camshaft position sensor to time spark and injector pulses. It has to see those signals during engine rotation, (cranking or running), to know when to turn on the fuel pump or pump relay.

Nope. I'm talking in generalizations because I don't know which your engine uses. All car models are different. Older Chryslers needed both signals for the computer to turn on the automatic shutdown relay. Later models, and some GM models need both signals for the engine to start, but once running, it will continue to run if one sensor fails during that drive cycle. Then, once stopped, it will not restart. Some GM engines will run on just one sensor but for starting, the computer picks which pair of cylinders to fire first, and it only has a 33 percent chance of picking correctly for a V-6 engine. If it picks wrong, you have to turn the ignition switch all the way off, then try again. Some engines start and run on just one sensor but injector timing will default to a backup strategy and fire in time with the spark plugs. Fuel distribution won't be the best, but the engine will run.

If you have a crankshaft sensor, it's somewhere where it can see a toothed ring or wheel on the crankshaft. If it uses a camshaft position sensor, that can be on a cylinder head where it will see a toothed wheel on a camshaft, or it can be a pickup coil inside the distributor.

The web site I use for service information is all messed up right now. It won't let me select a brand, model, or year. My best guess is you have a distributor and the sensor is inside it. GM put the ignition module in them too, then added the Engine Computer to the system to adjust spark timing. The sensor triggers the module; the module triggers the computer, the computer calculates the desired amount of delay to achieve perfect spark timing, then triggers the module to fire the ignition coil.