Stan
You can always stretch out your
nutrient changes, but you'll want to keep an eye out for signs of problems. If your plants are taking up 4 gallons a day, that's a lot for that size reservoir. That's basically the entire volume of the reservoir every 4 days, or almost twice the reservoir volume a week. But I'm not sure why your adding
nutrients daily, or in what amounts. If I were going to try and stretch out it out to two weeks, I would be replacing the water the plants take up with just plain fresh water every day. Except for the 7'th day, then I would replace the 4 gallons with about 1/4 to 1/2 strength nutrient solution (depending on how strong the nutrient solution originally was to begin with). Then only plain water every day after that.
If I were doing a nutrient change weekly in the same situation, I would most likely only add plain water. However if I were to add nutrients, I would only want to add it once, and about the middle of the week. Then I would likely only add about one gallon of 1/4 to 3/4 strength nutrient solution. Even then it would depend on how strong of a nutrient solution I started out the week with as to weather I added any, and/or how much. As well as how the plants look, temperatures, etc..