You won’t notice. If anything you will see an increase in mileage over the ROAD 5. I’ve been running a ROAD 6 on the front of my RT for 11,600 miles with a good amount of tread depth left. The rear is a ROAD 5. The two previous front ROAD 5s went about 10,000 and 11,000 miles respectively.
Michelin dropped the PILOT Road name for a series of tires something like 4 years ago with the introduction of the ROAD 5. The last PILOT Road in that series was the PILOT ROAD 4. If there is any question of the series look at the sidewall of the tire. Michelin prominently displays the name.
The front Road 6 now incorporates the 2CT+ feature of a layer of the hard center compound underlying the softer outer compound sections then the hard compound being thicker and wrapping around the edge of the tread. This stiffens the soft compound tread face at high lean angles thus increasing grip, wear, and feedback to the rider. For those reasons the Pilot Road 4 would be an inferior tire matched with a Road 5 rear in my opinion. The Road 5 rears already have 2CT+ construction.