Recognizing its potential impact on educational practices, this study aims to explore the impact of ChatGPT-aided lesson planning (LP) on enhancing 21st-century skills (4Cs: critical thinking, creativity, collaboration, and communication) among pre-service English teachers (PSTs). Utilizing a qualitative case study design, the research involved two PSTs conducting internships at different high schools. The study was conducted in two stages: the first focused on identifying PSTs' needs and providing structured instructional training on ChatGPT use, LP preparation, and 4 C skills; the second evaluated the impact through weekly discussions, reflection reports, observations, and semi-structured interviews. Thematic content analysis, aided by MAXQDA 2020, revealed that ChatGPT significantly enhanced critical thinking by aiding in problem-solving and evaluating multiple perspectives. Creativity was also fostered through innovative teaching strategies, communication was seen by helping articulate ideas and facilitate discussions, and collaboration was supported by enhancing idea-sharing and brainstorming sessions.