I am very surprised none of our Techies have responded. I will try even though I am a non-techie old man.
I am guessing each of your 4 current TV's is connected to one of your 4 current outputs (2 from your dual tuner and 1 each from the 311's.)
If you want to have the ability to watch different programming than any of the other TV's you will have to get another receiver which Dish will have to install.
If you want to watch the same programming as one of the other TV's (let's call it TV4 as an example) all you need is a $3.00 splitter and additional coax cable to run from the 5th TV to TV4. The splitter connects to the existing coax of TV4 and then feeds the 5th TV. You will have to install new coax connectors. Maybe you can find someone in your area who would be willing to spend an hour helping you.