There's still a few days to go before Bethesda launches Fallout 4 in all its glory, but the Fallout Pip-Boy app is now available for download on both Android and iOS. It works as an external Pip-Boy unit for the game, although you can't do much with it just yet since Fallout 4 isn't out.
You can, however, check out the menus, look at the maps and stuff like that. There is also a retro-inspired mini-game called Atomic Command. Everything else will be unlocked on Tuesday when Fallout 4 launches.