of course your mileage will vary depending on how you drive your car and conditions - lots of stop and go, lots of fast starts, etc.
However slipping in 4th gear seems kind of odd to me. That could be a transmission thing and not a clutch issue. I would expect you to have more slippage in the lower gears rather than the high one.
I would contact Specialty Z and see what they recommend. They sell RPS products and might be able to offer a solution.