4' is a bit high. Tegu's aren't much for climbing, but 4' wide would be great! 18"-2' is a good heigth, but I see you are going to put a waterfall in it. Definately a glass/arcylic front. A screen front wouldn't do well at holding humidity.
Just a suggestion, if you want good humidity in an enclosure, seal it all in, mount the lights internally (use a 3' ceiling for this, 4' would make it harder to provide a good basking area) and provide vents to allow air circulation though. For humidity in my enclosure, I use a ultrasonic humidifier I got at walmart, and attached 2" PVC pipe to the top of it, and it pumps in PLENTY of humidity.
I hope that helps 
>>i was thinkin about building a custom cage for my b&w. would 8 feet long and four feedt high be big enough. i am also thinking about putting a waterfall in it to keep the humidity up and he will also have a custom sleeping area under the cage so i can know where he is sleeping. does this sound like a good plan?
>>also should i use glass or screen in the front of the cage?