what matty was getting at is that your master cylinder will displace XX amount of fluid through full travel. If the internal volume of your pistons x travel x 4 exceeds this, you won't be able to get full piston travel at the calipers. (ignoring the whole front/rear bias thing!)
Not normally an issue, given teh original volume of the caliper doesn't count, only the amount of fluid required to move the pistons. However, what you may notice is that as you now have 6 piston fronts, with potentially a greater swept volume than the stock calipers, Y amount of pedal travel will translate into less piston (& therefore pad) travel. Or in English, you'll need to push the pedal more for the same amount of brake "pressure" - however, your more efficient braking system may couteract this from a "feel" perspective.
It's all a bit complicated and you should at least work out the "swept" volume of your new calipers vs the old ones to see if it's significantly greater.