Yeah, that was the problem. The site was configured with incorrect admin user information. The problem was solved by going to the user profiles and setting myself as a “super administrator” of the 4th site. Some how it had been created under a different user which was full of incorrect information. To make the site show up in your menu just go to “my sites” > “Sites” and you should see a list of all the sites.
On the right you can see which ones you are or are not listed as a user of. Find the ones that are missing your name, and hover your mouse over the “path” for that site on the left-hand side. You will see an edit button pop up, click it. On the new page you will see 4 tabs at the top, click “Users”. From there you just add yourself as a user for the site as well (be sure to select your role as an “administrator”. Then save. Your done, unless you want to remove an accidental user as well which is what I had to do, in which case edit their status however you see fit.