I ride in a 3, because that is the right size for me. I feel very comfortable and can easily swap between horses. if you are unsure, then one of the company fitters will help, definitely advisable if you haven't been fitted for one before.
I'm currently trialling a size 3, fits both me and J well. I'm 5'4" and under 9 stone and find it perfect size wise for me, J is short backed. I've ridden my previous horse in a size 4 but felt it was on the big size for me and was too long for J, this was an older Solution though and size 3 is a Smart Solution not sure if that changes the sizing though.
Size 4 eventer. 5 definitely too big even for my 6ft2 SJ coach. I'm 5ft8. I tried the 4 smart jump (I think) and some of the dressage saddles at the fitting and they were too small for me so there is a variation between models.
Size 4, I'm 5'4" and about a size 14. Size 4 fits fine , would be ideal if I lost a bit more weight but it's fine just now. The size 5 would definitely be too big for my mare and there's no way I'd fit in a 3!
Think it depends on your conformation and as I said it varies a little over the range too. The 4 or the 5 eventer was completely fine for my horse. I'm very long hip to thigh and I've literally never sat in a saddle that was too big before!