Coyote is correct go to your lumber yard with your picture and the size of the structure, explain to them what you are looking to do and they will have the beam (LVL) spec'ed for you at no cost. As far as installing the beam you could cut the 2 end post to rest onto of them. You could just add a 2x4 or the 4x4 you are removing up against the existing end ones. When they spec the beam they will also tell you what you need for bearing surface. Once done you could wrap it in pine to look as one. You have alot of different ways to make the connections to support the beam along with the finishing. You will also need to use some type of simpson connector as just toe nailing doesn't really cut it. You could also ask the lumber yard about the proper connection. When taking your measurement you will want the clear span in between the two corner post. If i would have to guess i would think you are going to need 3- 12" lvl or 2- 14" lvl. Let us know how you make out. If you go with a 3 wide member beam you'll have to change out your 4x4 over to a 6x6 for bearing. Good luck with your project. I am not sure where you are from in CT but Kelly Fradet is a great lumber yard to deal with.
