I didn't feel safe, and i know this probably slowed things down (cos i just wanted to go home)
I completely relate to that. For my first birth I am sure that I was doing well at home and was significantly dilated but things went backwards when I went in to hospital. I certainly felt stressed by some goings on and did not trust my carers. I absolutely believe that is one of the things that presented a challenge in my birth. (Good thing was that the IM support in my second birth was a totally different situation of trust and safety)

I am pretty sure that the hormones, although synthetic, act in the body in the same way and therefore set up the system for a more efficient subsequent birth. I am sure your midwife would be able to give you a researched opinion on this though. Plus, if you did get to 4cm, your own body was already doing it's thing. Surely this counts too??

Sorry, mine is only a lay person understanding so I don't think I am answering your question at all, but merely guessing and theorising along with you.