PGE/BE took me a LONG time to learn. The timing is something you need to get a feel for. There's no real way to know when it's been 4 frames. One thing that helps is knowing what too late or too early looks like. If you're too late mario won't move at all. too early causes you to trigger the loading zone and go through it. If you do it with perfect timing, mario will move just barely in the direction you pressed. Norm ally for PGE/BE you need to get it right 3-5 times before you can proceed out of bounds. It's also a good idea to go against the wall next to the loading zone in paper mode then go directly down (still in paper mode.) to get as close to the loading zone without triggering it before you start. Not only is the buffer hard, but the trick afterwards can be tough too. It's going to take great precision to learn this trick. Just keep trying. I'd say it took me several hours of practice at least even after I already learned the great tree cage skip. If you have any other questions after you get better at the goombella buffer just ask.
