The intensive English program is a mandatory subject taught by instructors under the Language Development Center (LDC) of Universitas Muhammadiyah Purwokerto (UMP). Because of the pandemic outbreak, this program is conducted in online learning. It will be a problem because in the 21st century nowadays, students should develop their 4C skills. This research aims to describe how the English instructors implement the 4Cs 21st-century skills in ELT online classes, to find out the problems faced by English instructors in implementing 4Cs skills in ELT online classes, to know the challenges faced by English instructors in online class, and to seek the impacts of implementing 4C skills in online classes towards the Instructor Professional Development. A case study approach was carried out in this research with five English instructors from the Language Development Center of UMP that were chosen as subjects of this research. Data were collected using observation and interviews. The data was then analyzed using the Creswell model of analysis. Based on the result, all instructors have implemented the 4Cs well in online learning, even though some criteria still cannot be implemented using online platforms. In addition, instructors deliver activities from warm-up to assignment to promote students' 4Cs skills. Instructors faced problems in implementing 4Cs skills such as internet connection, difficulty in eliciting students' inference, difficulty in promoting students' refining ideas, passive students, and students' lack of literacy. Moreover, instructors' lack of knowledge and difficulty conducting listening activities using Schoology also become a problem in implementing 4Cs skills. Furthermore, instructors faced challenges in online learning, such as schedule collisions and using Google Translate. Hereafter, the instructor can also identify the impacts of implementing 4Cs 21st-century skills in ELT online classes, such as developing creative and innovative teaching, enhancing the ICT use in the classroom, enhancing critical thinking and literacy, and fostering to design of adaptable teaching activity.