Hmm, nobody's adding the important flow component. I at least like to pretend I'm solid in IV. What gave me the ability to pretend I'm there is lots of runs on Clear Creek - lower and upper. I think it's great IV training. Lots of ambiguities though. Does solid IV mean you're solid on regular IV, or IV+ as well. I think that + means a lot in a rating.
I think there's an interesting distinction here that Danger is making. A lot of runs that are called class IV are really just a lot of class III with a couple sections that are class IV. Examples - lower clear creek, alto alto, Poudre Park. Then there are runs where the boogie water is class IV, and the main drops are IV+/V-/V. Examples: Black Rock on CC, Bailey, Gore.
To me these are entirely different classes, even though in some ways you could say they both fall into the class IV category. One demands that you be able to confidently navigate through class IV without or with minimal direction and scouting, the other, the class IV is the main event and you can set up safety, setup, plan, scout your line carefully.
OK, I'll give it a go, am in a buzz mood, but this is admittedly pretty ad hoc.
So in the easier category are runs like:
Alto-alto (per request):
<400 is scrapy, but doable III to IV-
400-600: fun, the inner canyon is class IV- to IV
600+: the crux definitely becomes solid IV, maybe closer to IV+ as it rises towards 1k(?)
The two main rapids are both easily scoutable and I would scout them first time down. The first one boats pretty easy, but second one has some potential to trip you up.
Lower Clear Creek: 400-600: IV
600+: harder IV
Lawson: <500: easier IV
500-750 medium IV
750+ harder IV (IV+ish)
Kermits: <600 IV
>600 harder IV
Dowd: One hit wonder, but good solid IV. I don't know the flows too well, but seems like 3.5-4.5' on the gauge probably rates a solid IV. Maybe IV+ over 4.5' or so?
Upper Mish: (like The Numbers

) very level dependent. 2.5 ft on the rock, III+ish. 4 ft+ on the rock solid to harder IV. When I first did Upper Mish around 4.5 I thought I was a solid class IV boater, but it opened my eyes.
Slaughterhouse: around 1000-1500, solid IV
Lower canyon Boulder Creek: 250-500. I think the Buttress is the definition of IV+.
Upper Animas: around 1000 is pretty solid IV with a little IV+ and a walkable V. High water I'm sure is another story.
In category II:
Here are runs where you can push up against the V- barrier, without going over (maybe some obvious picks here):
Clear Creek Black Rock: 300-450 you can even run Black Rock & Narrows and they're closer to IV+ at this level, but harder than IVs than you'll get on the above category runs at medium flows.
Bailey: 200-350 cfs (can do 3 of 4 Four Falls, and maybe Deer Creek)
Gore: 700 - 1300 cfs sans Gore & Tunnel
Escalante: low-medium flows. Take out before the inner Gorge and walk the cork screw thingy
If you're doing all of those well, then I'd say that qualifies you as a solid class IV boater.