I am trying to Support a little 8x12 pre-existing roof with 4x4's. there are 2x4 plates where I am trying to support, the 4" side down. I would rather not just toenail the 4x4 to the plate, and I know there are brackets, braces, plates, what have you, out there to buy. I have never bought these before so I'm looking for the name of the bracket I am looking for so i can find it in the store. Again, I need to attach a 4x4 post to a 2x4 plate with the 4" side going to sit on top of the 4x4 post.
Simpson Strong-Tie has a vast selection of product like this. http://www.strongtie.com/ has a downloadable PDF catalog.
What you're looking for is known as a post cap like the following and is used to tie a vertical post to a top plate. They're also available at Ace Hardware and Do-It-Best affiliate stores. Use the standard high-shear strength Teco nails to fasten them to both wood members. These are used because the bracket puts them under a shear load and standard length 16d nails weaken the wood and cause it to split.