Just finished a successful grow in 4 gallon SIP's. I used Pro-mix from walmart in the red bale, added perlite to approximate pro-mix hp.
Started off with Maxigrow then transitioned into Maxibloom. I was fertigating them every 2 days 1/2 gallon, straight water every 3rd feeding or so.
For some reason 1 of the plants wasn't uptaking the water as fast. I ended up watering it every 4th day. It was still soaking wet and heavy when I chopped it and inspected the roots. Seems like I get one plant that does that every grow, not sure why.
The 4 gallon size was perfect for 4 plants in a 3 x 3 tent. They didn't get too big but they filled up the space quite nicely.
Vegged them for 4 weeks then harvested at 10 weeks of flower, 80 days from the flip 120 days old.
My OCD liked those numbers.