The hole in the brass plug may be part of an air injection system. You may find another one a ways down the pipe. It might have a little plastic ball in the plug that will work like a check valve. They were used in the days before bladder tanks.
I don't know if a pitless adapter will fit in a 4" casing. You might want to make it 6" when you weld the top section on.
You might find a scrap yard that will have a piece of 4", 5", or 6" schedule 40 steel pipe. If you are really lucky they might have a concentric reducer with weld ends. Since there is no pressure in the casing you could just use a piece of steel plate to adapt the two sizes. You will want to be sure you don't get any weld material in the 4" diameter to reduce the clearance for the pump.
Check www.grainger.com for Stock No. 5YM17 for a pitless adapter. I think that one will also fit a 5" casing. Try to get a Grainger catalog from some business friend. They also have things like pressure gauges for about $4.
The black PVC may be a dark gray. That usually is Schedule 80. It may be worth saving if you can unscrew it while it comes up.