Ray, I feel your pain as I am currently going though the same process of replacing my steering control valve which also uses a 4 port module.
The 4 port steering control valves were used on the earlier model 318s whereas the 5 port valves were used on the later models. As I understand it, hydraulic fluid for all systems flows from the transmission through the steering control value to the steering as well as to the front and rear connections. With the 4 port control valves, the system runs in "series" while with the later 5 port control valves, the system runs in "parallel". The valves are NOT interchangeable without some serious re-plumbing of your tractor's hydraulic lines. In my discussions with JD, they have a "work around" to re-plumb the lines (changing a 4 port to accept a 5 port) but it has not been officially blessed by the JD engineering department, so they will not even guarantee that it will function properly once completed.
Initially, I found and purchased a used 4 port steering control valve on ebay and had it installed on my 318 only to find out that it was a defective unit - between the part and the labor, I wasted over $500.
I then sourced the service manual from Parker Hydraulics (the manufacturers of the hydrostatic control valve) with the intent of having my original unit rebuilt. In correspondence with Parker, all the parts are still available, but unless you are familiar with rebuilding hydraulic cylinders, if is not a DIY job. I took my module to a hydraulic repair shop where the mechanic noted the parts that needed replacing - this came to about $450 plus labor.
Alternatively, I approached Parker again to see if they still made the four port valve (I figured it they still make the parts, then they might make the complete valve assembly). Success, yes, Parker still makes four port control valves, but 1) they are expensive, and 2) they are custom made so lead times are long.
Just today, I ordered a new four port steering control valve and column - cost $800, delivery 6-8 weeks. (By way of comparison, JD wanted $1200 for a new 5 port valve)
The part number which I ordered is: HGF10002A11400
You can find the part number of your steering control valve on the one of the sides of unit very close to the end where the port connections are. It may be hard to find it the module is dirty.(although it should be the same as the one above).
If you want a copy of the service manual, let me know and I will forward it to you.