Clear Creek between Lawson and Golden above 700 cfs has some of the most continuous gradient class IV I can think of. Three V's along the way. Not high-quality wilderness, but about 25 miles and almost 3k elevation loss. In terms of guidebook named drops, there's probably 10 or so, but not every friggin' riffle has a name.
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Box Canyon on the Upper Sacramento near Mt. Shasta has the most high end pool drop, well defined IV - IV+ rapid concentration I have seen around those parts. The run is only 3 miles long but there are easily 15 really good rapids in that stretch if not more. You could add a lower stretch at high water and easily get 50 rapids in for the run, including the class III boogie, or do laps on the upper. Very scenic, you are in a box canyon running pool drop class IV, enough said.
Also in that area is the Cal Salmon which runs at a really wide range of flows and if you combing the Nordheimer and Butler stretches you can get 13 or so, miles of mostly IV - IV+ big water with a few V's thrown in for good measure.
That is an amazing map of Clear Creek. I think I've only heard of about 1 in 5 of those names. One of my highlights of this season was paddling Lawson to Golden. We did it just over 1000cfs, which makes the day more reasonable, and the entire Blackrock section class V. But the rest of the run is miles and miles of read-and-run class IV.
Another long mileage class IV is the Upper Animas. Ends with class V that you can skip. Big water style though, so not 'drops' per say.
For more creek style, I would nominate the Big South as having the most class IV drops. Although usually a class V run, lower flows combined with experienced guides and a less-experienced paddlers willingness to portage 4-10 times makes for a more manageable run. Most of the inbetween whitewater is class IV, and there are good scouts of a number of IV+ to V- lines in the easier rapids.
Escalante Creek could easily be sandbagged as one big class IV with a few hiccups. Which is why it makes such a great season opener.
The Shoshone River in WY is a sweet class IV river with good play, and it has okay length unlike a lot of runner ups, such as Numbers, Slaughterhouse, and Utah's Straight Canyon.
Rocky Mountain class IV is weak IMO compared to other states. Our rivers go from class III to V quickly. A Utah roadtrip at flood delivers predominately class IV. Idaho big water is often class IVish in a kayak... SF Salmon has over 40 miles in the lower canyon of top-notch class IV (okay, so maybe a class V undertaking).
Middle fork of the feather (devils canyon), way over a 100 class 4-4+ drops also has a few 5's thrown in, and it is 35 miles long. The run can be done in a day (I think lars holbeck did it in like 8 hours) but it is typically done in 2 or 3 days, with some of the best camping in cali the run has been called the best overnighter in the state.
O yeah and the upper animas in CO it is mostly 4 but is pretty continous for 20 miles.
And clear creek just seems like it is super continous but there is a lot of flatwater in there when you paddle from lawson to golden twice in a day, I think the most bang for the buck in from black rock down but that is just me
-Tom
Box Canyon on the Upper Sacramento near Mt. Shasta has the most high end pool drop, well defined IV - IV+ rapid concentration I have seen around those parts. The run is only 3 miles long but there are easily 15 really good rapids in that stretch if not more. You could add a lower stretch at high water and easily get 50 rapids in for the run, including the class III boogie, or do laps on the upper. Very scenic, you are in a box canyon running pool drop class IV, enough said.
Also in that area is the Cal Salmon which runs at a really wide range of flows and if you combing the Nordheimer and Butler stretches you can get 13 or so, miles of mostly IV - IV+ big water with a few V's thrown in for good measure.
Some of my favs. Travel safe.
Cal Salmon, beautiful river, amazing.
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In CA on the North Feather, there is a 1.2 mile stretch called Tobin. It is roadside, and you can do it 8 to 10+ times a day when they release, which they do one weekend a month all summer long. It is essentially a solid mile of 4+ rapids. If you did it 10 times, you would easily run 100 4-4+ rapids in one day.
being in my neck of the woods, is one of my favorites and in the spring I look at the gauge daily to get on it as soon as it's in. But for the top class 4 creek on the west coast it would be hard to beat the Opal Creek Run on the Little North Fork of the Santiam in Oregon. I know not what you asked for 'most class 4 in Co., N.C., Ca.'. But if the Box is in the running, Opal Creek has it beat hands down! Google oregonkayaking.net click on creeks or vids and behold.