Would you be able to leave a plant unattended for 4 weeks after a nute change? In an ebb/flow, 400w HPS, 7 weeks into flowering?
If it has been flowering for 7 weeks, and then you want to leave for 4 weeks? What kind of a strain would that be. After 7 weeks of flowering they should be ready to be cut down. I've never done it for that long, but I have done it for 2 weeks. Your E/F nute res should have a float valve on it ($8) and be hooked up to a water line. As your nutes are used up by your plants, the float valve will let more water in. So, your nutes might be a lot weaker after 4 weeks, but they would still be alive. The other concern is the growth of the plants as they reach for the light, and get too close to it. Although the growth spurt should be pretty well over in the late flowering phase, and you will not have to flush them when you get back. sn