I don't want to sound like a jerk, but it takes along time to learn how to paddle class IV whitewater. It's not somthing that you just start doing after rolling in the pool. If the Poudre is convenient, I would suggest that you start your river running on the Filter Plant Run (between the confluence to a pullout 100 yds above the lowhead dam. If you like that, try the Bridges section. Then try Lower Mishawaka and then Upper Mishawaka. If you're successful, you will probably be ready for the Rustic Sections. Rustic isn't really that difficult, but it's probably better that you spend time on the lower, warmer, more often run sections. Keep in mind that I'm not saying you should immediately go from one section to the next. Spend some time learning the basic paddling skills like eddying, edge control, ferrying, etc. When I was new to the sport, I used to ask people if I could tag along on stuff like the Filter Plant. It's a good opportunity to meet new people and pick up some skills that you'll use paddling in the future.
I should probably say that my first kayak run was down Cataract Canyon III+ at low water, but I had a lot of experience reading rivers prior to learning to kayak. I was a little scared the whole way through the rapids. Yes, it can be done, but it's better to learn stuff in an easier environment.
Best of luck in learning how to paddle!