wow that sounds like a great pond you've started. Since you have fish that large already it's not too early to start taking some out. If you wait you are going to have things stunted quicker and are going to be working an uphill battle. If I were you go ahead and start a slot limit. Use a digital scale and take out anything under a weight that you determine to be appropriate. I would not hesitate to go as high as 30 pounds an acre. Focus on removing the smallest more so than the higher at your limit. Take out all males if possible. You will always have males replaced by reproduction and they are going to top out at 3-5 pounds max. Maybe where you are a little larger but the females are going to be your trophies. You can tell by the vent for the gender. round perfect circle is a male, elliptical vent that is possibly distended and red is a female. I think if you can't tell go ahead and remove anyway. You are never going to be short on fish. The more you take out the more food you have to grow that original stock that are already 4 pounds. Again sounds like you have a real good set up already but trust me, it will quickly get out of control and stunt if you don't remove some. This advice is of course if you want trophy bass. a lot of people on here focus on the relative weight idea and how long the fish is in comparison to it's weight. That's great and all but no matter what you gotta think the more you take out the more food is in there for the remaining fish. personally I would not remove any bluegill for a few years and even then do it very minimally. that's my 2 cents for you. Good luck this summer!