Yes with Clitheroe it's all ranked. Ranking comes into play for those out of the catchment area. If your in the catchment area then all you have to do is pass, you could have 20% less then the person say 20th on the reserved list but still get in as you live in the area.
Now out of the 120 places that they have offered, there could be about 4 or 5 in their who have not put Clitheroe as first choice, or decide to go elsewhere to an independent school or something. Which would then mean that the top four or five would be offered places in rank order. So if only one drops out then the person first on the reserved list gets in. Like you rightfully said, the parents of the child 2nd or 3rd on the reserved list may drop out, which means my son would go up on the reserved list.
We know last year they took 11 from the reserved list, and the year before a lad in our area, who was 7th on the reserved list got in. Everyone at my wife's school is saying that being 4th on the list our son is pretty much in. But again there is no guarantee as they could easily end up just taking 2 or 3 from the reserved list this year. We only find out in March.
Yes nothing is guaranteed til you get that letter but 4th does sound positive x