My ob also said the 4cm thing but I have always gone from when contractions were regular and painful. Having said that they were never really regular until I had syntocin.
My waters broke at 11pm tuesday and I started having 5-7min contractions by 12am that night. Went to the hospital 10am wednesday but was only 2cm dialated. Laboured all day and night with the contractions getting to 2mins apart but I still wasn't dialating much and had had no sleep despite my doctor giving me a sleeping tablet, then at 2pm on the Thursday I was 6cm so they put me on the highest dose of syntocin and it was predicted I would have my bub at 11pm. Well at 11pm was only just fully dialated and didn't actually have my baby until 2:23am Friday morning after two hrs in second stage Olive wouldn't engage so then an epi and forceps delivery. So I say that my labour was 50hrs and my husband and mum who were there can verify it if nobody believes me. I also think it's a good idea to write your birth story soon afterwards as it not only helps the healing process which I needed but you also have a more detailed record of your most amazing achievement and can also reflect upon it when and IF you decide to go back for more.