Mellor, who played in the recent victories over Leeds and Wigan, and hooker Conroy have penned new one-year deals, with Bates and Brooks committed for two more years.
Coach James Lowes said: “Alex, Nathan, Sam and Adam all have the attributes and potential to have big futures in the game.
“Alex and Nathan have a fair bit of Super League experience, while Sam and Adam have been building within the Academy this year.
“Their attitudes have certainly impressed me and, although they all have a long way to go, I am excited about working with them going forward.”
The signings takes the number in Lowes squad for 2015 up to 18, although one player who won’t be playing for the Bulls next year is three-quarter Sam Wood, currently on loan at Dewsbury, who the club have confirmed will leave Odsal at the end of the season.