Forum:Hobbits in the 4th age

Hello! I am a huge fan of all the books, but I noticed that the WIKI here has information regarding Sam, Merry, and Pippin (and many others Hobbit and Non-Hobbit alike) and what became of them after Frodo departed to the grey havens. Information such as Sam's 13 children, Rosie's Passing, and Sam's eventual departing to the undying lands himself. I was just wondering where this information came from. Was it in another of Tolkein's books that I haven't yet had a chance to read? Thanks to anyone who has an answer for me!