Agreed there should be good focus on fundamentals before small circ is done. There's so much depth you can find even just doing embryonic breathing - establish that path of least resistance before going on to SC.
4 gates is laogong/yongquan breathing. Stand in zhan zhuang, inhale from the palms and feet to the dantien, exhale back to the dantien. Standing is better to learn reverse breathing imho - you need to do a bunch of work on the psoas muscle to do reverse properly, that sensitivity can be gained by doing natural breathing and learning to descend the psoas/diaphragm connection as the beginning of the inhale, then you can breathe very deeply, eventually good breath mechanics with the psoas and perineum leads to breath going so deep it externally disappears. Then you really know how to use the psoas muscle. Then utilize in reverse breathing, dont worry so much about what the front of the stomach is doing, make sure you are descending the psoas-diaphragm connection to start the inhale and you should avoid having too much back pressure from the motions. Psoas is also right in there running the kidney sinew channel down to the feet, so this provides physical basis for the yongquan breath.