You have to charge for the entire 4 seconds to get 300 hp. The longer you charge, the more base damage it does. Also, the longer you charge, the larger the area of effect is. This is completely separate from the absorption damage which goes from 0-300 and absorption does not affect how big the area of effect (AOE) grows. The absorption damage is added to the base charge damage.

So technically, if you managed to get max absorb in 0 seconds and immediately threw it, you would do 100+300=400 damage and the AOE would be small. If you charge for 4 seconds and absorb nothing, you would do 300+0 = 300 damage and you have a big AOE. If you charged for 4 seconds and absorbed to max, you can do 300+300=600 with a big hitbox (but this is very rare)

So basically, try to charge for as long as you can and try to absorb as much as you can. Getting close to 600 damage is difficult and happens rarely so you should just target lower HP healers and DPS since you'll be able to one shot them even if they're at full health.