The most heavily promoted advertisements in 4 medical journals with the highest readership in both countries will be chosen on basis they are available more and to many doctors.
The sentence is trying to say too many things making it difficult to grasp. How about having the points in a list like this:
Advertisements shall be chosen on the following bases:
1. Level of promotion in the four medical journals
2. Size of Readership in the two countries.
4. Circulation to doctors.