2021: Kotaro Shimbara made his F4 Japanese Championship debut, competing from the third round with the RSS Team and finishing the season in 14th place.

2022: He joined Zap Speed for a full season in the F4 Japanese Championship, concluding the year in 20th place with one podium finish.

2024: Shimbara signed with HYDRANGEA Kageyama Racing, where he claimed his first victory (following a competitor’s disqualification) and secured one pole position, finishing the season in 3rd place overall.

2025: Shimbara continues his journey in the FIA Japanese F4 Championship with HFDP with B-Max Racing Team, following his success in winning the Honda Racing School scholarship.