I am building a cat enclosure connected to the house underneath the outside roof which will have an extended roof for the cat enclosure. It is 4 foot wide by 17 foot long. *I was wondering if I could use 4 x 4 post as a Header or do I have to use 4 x 6 post for header for the rafters (2 X 4) *12 inches a part to set on. *There will be for four 4 X 4s sitting 4 feet apart on a existing deck. *After this project is completed I will screen in around the cat enclosure.