Go here and measure your pond to know it's size for sure.
http://acme.com/planimeter/ Depending on the amount of weeds you want to regrow in the pond after the herbicide treatment, you might need more or less Triploid Grass Carp. For complete control, 12-15 per acre would do it if you have a lot of plants. So, you can see how the difference between 3 and 4 acres is critical.
It's also critical for applying herbicide. You also have to get a good idea of the depth profile of the pond to correctly figure out how many gallons of water are in your pond, especially with Fluridone. If you think you have 4 acres and only have 3, and apply as if you have 4, it will cost a LOT more. You can't spot treat with it, the whole lake has to be treated, and you should keep the concentration at 90 parts per billion for a minimum of 45 days.
You could also use Diquat with a copper kicker and a surfactant.
