The first number in the power area is the total power needed to fire, the second is the amount of power drawn by the capacitor of the weapon each 5 seconds(which is also the number after the C in the default weapon name). As powerplants are rated by their production in power per 5 seconds you need to match your powerplants to the total capacitors of your weapons so in this case with capacitor 4 on 4 weapons you need 16 power.
As an extra point, when designing your weapons make sure to compare the capacitors available to the power needed to fire. Particularly with larger weapons, as you can often get away with using a smaller capacitor while maintaining the same rate of fire, which also results in a cheaper weapon as well as not needing to provide as much power from power plants. For example if you have researched your capacitor up to 6 it wouldn't make your 20cm lasers fire any faster than just using a capacitor 5 while just making the weapon more expensive.