They're not 4x4's, but two 2x4's screwed together. Almost the same strength. I'm pretty confident they'll be fine, especially considering the 180 next to it has 6 legs made with single 2x4s and the stand is rock solid.
The tank has been put together. I've got pics I have to offload from the camera showing the builders assembling the tank. I'll post them asap.
The main builder will be back in a day or two to do the bracing and to set up the overflow box in the tank. I'm hoping to do a wet test this weekend.