I don't see that this is an issue at all. People tend to overreact when it comes to what their floor can support. If you're spanning 4 joists that are perpendicular to the tank's front panel and you are located close to a weight baring wall you have nothing to worry about, (assuming you have 2x10 or 2x12 floor joists.) The 4x4 supports you added really aren't necessary for anything other than peace of mind.
If it puts your mind at ease, I have a 195 gallon six foot tank sitting on (4) 2x10 joists with no additional reinforcement to the floor system. My tank weighs 350 pounds, my stand weighs almost 200 pounds (overbuilt), and my sump weighs almost 250 pounds full. With the tank filled, I'm loading the floor almost 2,700 pounds in a 6 foot area. As long as your stand is built well and the tank is level and plumb you shouldn't worry too much. When people calculate loads they tend to read them wrong. The way it was explained to me from a structural engineer is that you have to fill the entire floor with the maximum weight allowance per square foot...not just the area where the tank rests.
HTH
