Unfortunately Wenatchee is on the Eastside of the Cascades and the majority of the water in March is going to be on the Westside of the Mtns. I have a fulltime job during the week so it is doubtful that I would be able to just screw off and have fun for 4 or 5 days, but I'd love to go boating for a day or two on the weekend.
Some runs that aren't too far away... Check out the AW page or a guide book if you have one.
Tumwater Canyon on the Wenatchee R.(1 hr from Wenatchee)- You said you only want to boat Class IV... the Wenatchee will probably be running 5 or 6K in March and I believe that is the Max suggested flow and at that level it will be solid, pushy, big water Class V. (You'll have to find another guide for that one!! he he)
If the Wenatchee is above 10K there are is some superb play on the section past Leavenworth. (45 min from Wenatchee)
The Skykomish and the NF of the Skykomish (2-2.5 from Wen.)- the NF is pretty straight forward class IV other than the first couple rapids which are a bit more difficult. (reminds me of the Numbers) Don't run the NF unless the Main is above 10K. The Main is mostly class III with one rapid that pushes IV+. Anything above 10K the main is IV with one V.
The Green River Gorge (1 hr from Seattle) Gorgeous class IV run. There's a lot of wood this year so be careful especially at flows of 3K and up.
The Sultan (Near the Sky) another great III-IV run.
The Foss (Near the Sky) another great IV run.
The Tye (Near the Sky) another great IV-V run.
If you have a chance to make it further south the White Salmon and the Wind rivers are spectacular!!! The White Salmon has 30 miles of awesome Class III-V+ rapids... fun for everyone. The Wind is a bit of a step up from the section of the White Salmon you'll probably run. Steep class IV boulder gardens with some nice drops and one mandatory portage.
Sorry for rambling... tired... shoot me an e-mail when March gets closer.