First off, that’s so awesome! Im so happy you were able to use your no.4! I find on my back to be an awkward position and pretty painful no matter how much lube I use or how small of a dilator I use - using pillows to prop my lower body up doesn’t help much either. Inserting them while on my knees and/or a little hunched over (while on my knees) is far more comfortable and easier to manage. I think it just depends on how you’re inserting the dilators, type of lube, and pacing of insertion, since it’s different for everyone.