Which version of the Phantom do you have? The LED status indicators are listed in each manual.
One green flash followed by 3 red flashes means you're in GPS mode but the Phantom sees less than 5 satellites. If you let it sit for a minute that should eventually go down to two red flashes, then one, then none (just repeating green at that point)...once you're down to 1 or no red flashes then you have enough satellites locked for GPS mode to give you stable flight.
One green flash followed by 3 red flashes means you're in GPS mode but the Phantom sees less than 5 satellites. If you let it sit for a minute that should eventually go down to two red flashes, then one, then none (just repeating green at that point)...once you're down to 1 or no red flashes then you have enough satellites locked for GPS mode to give you stable flight.