A 4'x15" cage will last the Anaconda to 8', maybe. It will need a larger cage overtime, male or female. 15" wide is deffidently not going to be big enough, you are going to have to have *at least* 6' x 30" for a male, may still need to go bigger, and 8' x 30" for a female overtime. The temps should be at about 80-85 all around and a basking area of 90. I don't know how you have the heat pad, but do not leave it exposed to the snake at all. If you order from lllReptile, you are going to end up with a wild caught almost guarenteed. Cheap is deffidently not the way to go with Anacondas, what you pay is what you get for the most part.
You go about feeding it the same way you would go about feeding any other snake if you can get it to eat. Some can be very difficult to get to eat, and some wont, but don't order one with the hope that it will and just expect it to, be prepared for it not to just in case. Wild caught snakes tend to be a lot more difficult to tame, and it's just a risk I suppose, most will, but I have heard of some that just wont do it. C/b is deffidently the way to go, but just remember that if you do something that this snake doesn't like, no matter how tame, it will show you somehow. I have an 8' Male green that is like a puppy, he has snapped at me one time before by me trying to move him when he was trying to sleep, but that was the only time. I have also had one that once the cage door opened, it came right out with it. So no matter what, be prepared for a bite, something that you should do with all larger snakes though.
They grow at all different rates, but you will probably have to change your cage within the year, or at about a year 1/2. Once the snake hits 7-8' you will have to change yours. For the most part they grow fairly fast though, so be prepared for that. Feel free to ask any questions, there is a lot of info on these things that is something that should be known before owning one, so ask as much as you want!
Oh yeah, and be prepared for frequent cleanings and disinfectings. There are weeks when these things go to the bathroom everyday! Not a big deal as a baby, but once it hits 8' , it becomes a big deal, lol.
~Ben