How does the PCM control the fuel injectors

That sounds fine, but the wiring diagram in the shop manual does not match that at all. You can see the two pins feeding to the two groups of injectors, pin 59 feeds the odd numbers cylinders, and pin 58 feeds the even numbered cylinders, then on the other side of the injectors you can see all the grounds are tied together. Makes no sense to me at all.
What got me here was I wanted to run a test by turning off each injector to see if the RPM's drop off on each cylinder. They are so hard to get to I was going to add 8 N.C. Momentary push buttons to my breakout box so I could momentarily interrupt the signal to each injector without physically removing each connector. I was shocked to discover I only had two injector pin outs to work with instead of eight. So that ruined my bright idea.

I should add, that even thought this is technically a 1996 van it was early on the assembly line and never got the OBDII upgrade. I has the EEC-IV EFI ignition system from 1995 and earlier.

If the injectors are connected/wired together in groups of four how is each injector opened individually at its proper time if all four are wired together?

That makes sense but not in relation to the ignition system. This van still has a conventional distributor with a rotor that is sending sequential individual spark to each cylinder. So if the cylinders are getting fuel in simultaneous companion groups of four it would mean fuel is being injected at times when some cylinders are not receiving a spark and the piston is in the wrong position. How can it work if the spark is sequential to the eight spark plugs but the fuel is sent to four companion cylinders? Just does not add up for me.

That is the same wiring diagram I have. It still does not explain how it works. You can see that all the injector grounds are tied together and all eight cylinders are controlled by only two wires from the PCM. If four injectors are controlled by the same two wires how can each one inject fuel at the correct moment? Wouldn't all four inject fuel at the same time? That would be okay if all four cylinders were firing at the same time, but they are not. The distributor is firing them in sequence, and each injector must operate in a matching sequence. So how is this possible?
If half the rail is charged at a time what controls that charge? The only thing I see is the fuel line from the gas tank, a fuel pressure valve controlled by the amount of vacuum in the intake manifold, and the pipe splitting off to the two injector rails. There does not appear to be anything that would allow the rails to have separate charging, and nothing in the wiring diagram to indicate any other fuel controls.

I guess so, but it will continue to bother me. On all the other size engines that year the PCM had an individual pin/wire for each injector. Do not know why they chose the 5.8 to do something radical. I encountered another odd ball in this van. The MAP outputs frequencies to the PCM instead of voltage. That had me stymied for a while. Is it possible they did the same thing with the injectors, where each of the four in a bank respond to a different frequency? If I ever find out how it actually works I will be sure to let you know.
Thanks

I finally figured it out. The key is that fuel does not necessarily need to be injected at a precise time. It can be injected at any time during the power stroke. So what is happening is that, at any given interval, four cylinders are simultaneously in a power stroke and are all injected with fuel at the same time, then the distributor fires those four cylinders in sequence as they each come to the top of their respective compression stroke. Not very efficient in my mind but I can finally see how this system works.