At the Donne the gradient eases (12m/km) and class IV water lasts for a kilometre or so, then it's into class II and III before Milford Lodge.
To get to the put-in: go through the Homer Tunnel, drive down the Cleddau valley to The Chasm carpark, pose for pictures among the hundreds of tourists. Scramble out the end of the carpark down to the river. If you're getting in at the Donne River, either bash down the Donne River itself for 150m, or drive 500m further towards Milford from the Donne River bridge to where the river runs close to the road.
To get to the take-out: drive down the valley and turn off at signs to Milford Lodge. The Lodge is a backpacker hostel with camping sites, a shop, showers and washing facilities if you need these. (There aren't any others in the area).