Since lots of works has already been done in terms of battle balance for the 1.0, its highly unlikely that you would see such a thing for then. Part of the damage equation of ranged units is the maximum ammo they transport, so everything would need to be re-balanced if it was changed.
With scarce ammo for javelineers, the problem arise with the accuracy/ammunition equation: you can easily waste most of your precious ammo on the ground and on shields.
Low ammo would also make javelineers "melee units" (0 ammo left) more time, their melee stats are actually horrendous (except some average ones) so they would be fodder during that phase.
And if javelins, and per extension, ranged units, are not a "profitable" unit to use on the battlefield (in terms of casualties done on the opponent relative to how much they cost and how easy/hard they can be used) then we largely remove them from the armies of Rome 2.
That being said, I would like it if we found a solution to have realistic ammo count and keep the precious balance of battle.
One remark though, dedicated skirmishers could certainly transport more than 4 javelins at times. Is 8 or 12 javelins per men that unrealistic? (not all in their hands of course, but in bags for example, if such a thing existed already back then

)