I think you'll probably have to head over the pass but if you're willing to head over Steven's pass there's the Rapid, the Foss, East Fork of the Miller, and Money Creek that can be had near the town of Skykomish and should be in or close to in.

If you want to take a quick hop over Blewett Pass the North Fork of the Teanaway might be in. Not IV-V but if the gauge is over 1K it's a good continuous gradient III+ with enough seriousness to keep most IV-V paddlers engaged.

Upper Tumwater (takeout at Swiftwater) is big III-IV rather than IV-V at these flows but can be a fun way to mix things up.

500 cfs is probably the minimum that is going to be any fun. It will make ingalls pretty scrapey and low. Above 1000 it gets really fast. But its runnable way above those levels.