Bulbs

will 4 100 watt light bulbs work for 2 plants ?

at the start, you will need more light later on.
to reach the sides of your plants, 4 will not be enough lumens

About how many more lights and what stages will I need more light ?

once there weeks into veg, you will need alot more for a good flower.
In my box I have 10or 12 23watt cfls and 3x 45watt cfls
you need enough to penatrate the canopy and light up the sides.(later on)
Light extensions or droping lights down to the side will help later on also.
Seedlings will be happy with the amount of light you got there.