Many people still use the 4 pin hubs/wheels on splined axles because they have some older wheels that don't have removeable centers or the centers are on an oddball bolt pattern which makes it impossible to find a 31 spline wheel center to put in its place. After spending all the money they spent to convert to a 31 spline axle they wanted to find a way to retain all their wheel inventory. I have several buddies still using 4 or 6 pin wheels on mini sprints and 6pin wheels on midgets.

They may be heavier, but they are for sure cheaper than buying more, newer wheels. I have one buddy who still uses 6 pin on the midget because people are constantly giving him free wheels that they refuse to use anymore.

If I was in your position here is what I would do. Get the proper pressure plates for now so you are sure your set-up is correct. Make sure the plate has the same taper as the axle nut and the tapered spacers so they stay snug. Drill a 1/8 inch hole thru the end of the axle and put a safety pin in it. Having that safety pin will subconciously keep you from overtightening and can save you a lot of damage and embarrasment if you do forget to snug a wheel down.

Keep an eye out for used 31 spline wheels. If they are straight and have good meaty spines left in them and can be picked up for half of new price or less grab one here and there when you find them. As you damage your current wheels always replace them with splined wheels versus pin wheels... (pinwheels..haha)...Unless somebody offers you DIRT CHEAP or FREE used 4 pin wheels.

Once you get to the point that you have a few splined wheels you will find that you aren't really using the 4 pin wheels very often. At that point you can take the 4 pin hubs you still have, replace the drive pins with bolts, and bolt them into the wheels you are most likely to use, and treat them like a splined wheel for a spare/emergency situation....if your hubs don't have the taper in them you will still need to use the pressure plates so just sandwich them in with the bolts.

And as you start this process pay attention to what brand/bolt pattern your 4 pin centers are. If any of them are Weld or Sander...or if any of them use the same common bolt pattern as Weld or Sander, you have the option of picking up some used splined centers to put in them. You also have the option of using the halves as replacements should you bend any of the halves on your splined inventory.

With some hard core shopping around, a trip to the hardware store, and a little bit of elbow grease you would be amazed how inexpensively you can have more modern, lightweight wheels on your car. When I built my first sportsman midget 6 years ago I was dead set on having splined rears and direct mount fronts. I ended up finding a complet set of 6 pin beadlock Weld 3 piece wheels for $50 each. ( 2 rears/ 2 fronts). I picked up 2 used centers at the Chili Bowl for 20 bucks each and found a set of older DM hubs for 40 bucks that were intended to be used with one piece rims. I drilled relief holes in the front hubs for pinch bolts to recess into and spent a couple hours replacing/removing the 6 pin centers. I spent $280 total (plus a couple bucks for some new pinch bolts at the hardware store). Then later I was able to sell the 6pin centers for 10 bucks each.

So for less than 250 bucks I had beadlock wheels on all 4 corners, mud covers on the rightsides, splined rears and directmount fronts plus front hubs, and had pretty much the same stuff as the guys who buy brand new every year. When you consider a new Beadlock RR wheel costs at least $380 you can see why I was willing to shop around and use some elbow grease.