I skipped Marys Peak because I had done that one already, but with the gate open on Prairie my brother and I did the other three starting from Corvallis in about 9 hours while taking our time sightseeing and stopping for lunch in Veneta.
We started at 9am and finished around 6pm, so there was plenty of daylight left over, yeah four should be easy.
Sadly you still needed to do Mary's again to earn the hallowed honor of "4 Banger". Anyway, that is still a decent haul, 3 in a day. congrat's on your effort.
Dean, thank you for another excellent route description.
The "unsigned paved road" off the Alsea-Deadwood road is now (2015-10-08) signed "PRAIRIE MT RD".
Also, about 0.1 mile before the gate the road forks. The uphill fork (on the right from the north) leads to the quarry and gate. This side road continues back down to the road it left to the south.