Objective To evaluate the functional and aesthetic evaluation of external fixator lengthening through plantar approach for fourth brachymetatarsia. Methods A retrospective analysis was conducted on 20 patients (23 feet) with fourth brachymetatarsia who met the selection criteria between January 2016 and January 2024, including 3 males and 17 females, with 8 left, 9 right, and 3 bilateral cases. The mean age was 24.7 years (range, 14-51 years). The preoperative metatarsal shortening length was (13.8±3.2) mm. The preoperative American Orthopaedic Foot and Ankle Society (AOFAS) forefoot score was 79.5±3.9, the visual analogue scale (VAS) score of appearance satisfaction was 1.7±0.8, and the appearance index (AI) score was 13.6±0.9. All patients underwent external fixator lengthening through plantar approach. The lengthening length of metatarsal bone, lengthening ratio, healing time, and healing index were recorded. Functional outcomes were assessed using the AOFAS forefoot score, VAS score of appearance satisfaction, and quality-of-life impact with AI questionnaire. Results All 20 patients were followed up 14-55 months with an average of 36.3 months. During the follow-up, complications occurred in 4 cases (17.4%), including 2 cases of metatarsophalangeal joint stiffness, which had no significant effect on the function and appearance. Delayed union of osteotomy occurred in 1 case (healed at 12 weeks after operation). Pin loosening occurred in 1 case and recovered after outpatient reinforcement. No complications related to plantar scar occurred. At last follow-up, the lengthening length of metatarsal bone was (13.9±3.1) mm, and the lengthening ratio was 25.8%±5.6%. All cases achieved bony union, with a mean healing time of (64.3±12.5) days and a healing index of (46.9±4.8) d/cm. At last follow-up, AOFAS score was 98.9±2.1, the VAS score of appearance satisfaction was 9.3±0.7, and the AI score was 0.6±0.8, which significantly improved when compared with those before operation (t=27.398, P<0.001; t=32.994, P<0.001; t=56.135, P<0.001). Conclusion External fixator lengthening through plantar approach is a safe and effective technique for fourth brachymetatarsia, achieving satisfactory functional and aesthetic outcomes.