4 watts per gallon is very high light intensity for a 55 gallon tank. And 8 watts per gallon is a getting to be pretty high light intensity even on a 10 gallon tank. It isn't too much if you mean it will kill the plants or fish. But, if you don't really keep up with the maintenance of the tank, prune every other day, make sure the CO2 concentration in the tank stays the same day after day and is about 30 ppm, and you make sure you are dosing every nutrient so as to avoid a shortage of any nutrient, the plants will not grow fast and algae will soon take over.