Employers mentioned in 4° bis of article L. 5424-1 who have exercised the option mentioned in 2° of article L. 5424-2 shall pay, in addition to the contribution provided for in 1° of article L. 5422-9, for a limited period of time, a specific contribution based on the gross remuneration of their statutory and non-statutory employees up to a ceiling, under conditions set by decree.