All depends on how many plants you plan on using in your 4x4 and how long you would want to veg for. 7 gallon is around 26L so I reckon you could squeeze 4 of them in a 4x4 and is the largest size I would consider for sure. I normally use 4x 23L for soil in a 4x4 or 4x 20L for coco. Now there are other ways for example I have just done a single 20L pot with coco and scrogged and it takes up the entire tent with around 40+ main heads although veg time was very long. In a country without plant number rules I would still go to 16 plants in 7-10L pots as the veg time would be smaller allowing more cycles per year. In Spain I even did 25 plants in 7L pots with only 7 days veg and could still pull 1-1.5 lbs (5-600g) per 4x4. Hope this helps pal👍