The actor revealed the casting via Twitter, but initially mistook the game for a Battlefield spin-off.
"Just signed on to do the new AO4 video game for Electronic Arts. Some splinter from Battlefield. Finally, my kids think I am cool again," he wrote.
Flanigan later released his error and tweeted: "Actually, showing how distinctively uncool I am, I think I have those games mixed up. Thank god my kids don't read Twitter."
Army of Four is rumoured to be the next entry in the Army of Two series, featuring four-way co-op gameplay and DICE's Frostbite 2 engine.
EA is yet to officially announce the game, though it has been linked with an unveiling at next week's Game Developers Conference.
> Read Digital Spy's review of 'Army Of Two: The 40th Day'
Watch a trailer for the original Army of Two below: