does anyone have a driver or a company they would recommend to drive 4 of us ?
the logistics are confusing. You want the driver to take you to PL, wait 6 hours, and then drive you to Split? I suppose that you could get this, but I doubt it will be cheap. For some, that's not important.
Have you looked into buses?
If you're not able to find a suitable private transfer or you decide the price is too high, consider renting a car for the drive to Split. Driving in Croatia is easy and the roads are good. The drive would be mostly expressway, except for the detour to Plitvice. With a car you'd also have the option to add an overnight at Plitvice, thus avoiding the worst of the crowds as you tour the park.