The below linked article is from CenterWatch which closely follows clinical trials. Note that according to them 70 to 90% of all drugs in phase 3 trials advance to phase 4 . I have also linked a site that tells about centerwatch so those inclined can get a better idea of how they fit into the clinical trial picture.

Quite a spread from the other site that claimed a 25-30% advancement rate after phase 3. Makes one scratch his/her head.

https://www.centerwatch.com/clinical-trials/overview.aspx/

https://www.centerwatch.com/