I bought a ball spinner and have been trying to figure out what I should bring to have a well rounded arsenal, too. My current is
Hyroad Pearl (oob)
Hyroad (4000)
Marvel Pearl (oob) or Fusion (4000)
Thug Unruly (2000) or Optimus (2000)

With the Marvel, Fusion, and Optimus being so similar in RG and all being Medium/Heavy rated coverstocks (and the Hyroad too for that matter) I've found only a slight difference. For example, The Marvel and Fusion can play the exact same line at the same time but the fusion checks up earlier and may be a half board higher into the pocket because of the 4000 surface. The Hyroad is great to blend out the pattern but its a very slight "step down" from the Marvel Pearl. Maybe a 1 and 1 at most. The Optimus and Marvel were Identical in terms of boards covered but the Marvel seemed to skid further before picking up the midlane but was a smoother transition which I liked for reading the lanes. I changed the Optimus surface to 4000 and didn't see much difference. Finally a move to 2000 gave me a slight increase in boards covered and now fits more closely to the Thug Unruly with a much less angular shape.

for me from top to bottom I have less then a 3 board difference in all these balls. Going from low RG to higher RG.. or 1500Polish to 2000grit. From Solid to Pearl. The most tangible differences are in the shape of the shot and energy preservation.

If I start going over under with my Marvel Pearl, they hyroad blends it out nicely and keeps me in the pocket.
If I'm playing a deeper line with the marvel but it seems to have no energy left at the pins or can't seem to turn the corner, the energy retention and midlane read of the Thug Unruly is great.
If I'm blowing through the rack leaving greek churches or 9 pins with the Marvel, the Optimus helps burn up some of that energy and hit at less of an angle.
If the lanes are so chewed up that I can't seem to retain energy or keep it to the right of the head pin the Hyroad pearl allows me to play for a little bit longer in that area and still retains good energy.

Its my long winded way of saying, I don't think anyone could just look at the specs of the ball and give you good advice. It ultimately comes down to how you roll it and what situations in which those balls excel..If two balls are doing the same thing, then your only choice is to make surface changes to try and separate them. You obviously need something for Dry dry lanes.. and you need something for fresh of flooded lanes.. so those two are easy. For the remaining two I would look at them more from the difference in angularity or speed of transition than through RG numbers or Surface numbers.