If you are running Class D amps with higher efficiency ratings than 50%, you can get away with about 20% more wattage than the graphs indicate.
The cost difference between 4 ga. and 8 ga. is minimal over the 16 ft. run, and the connectors, fuse holders etc. aren't much more either.
The work involved with running either gauge is the same, so I would highly recommmend running 4 ga.
Otherwise you have no room to upgrade, and even at lower wattages the bigger gauge cable is the better plan to prevent stress on your system under heavy usage and/or high temps.