Privateer post free Grind expansion
From their website:
Take your Grind games to the next level with Mosh Pit, a free expansion that lets you and three friends face off in the biggest Grind arena yet. The four player Mosh Pit rules and Mosh Pit board can be downloaded below in both full and tiled versions for easy printing.
Assemble your team, hit the grid, and get ready for four player metal mayhem.