Selection of the lowest instrumented vertebra (LIV) in Lenke 5 and 6 adolescent idiopathic scoliosis (AIS) is controversial, as it requires balancing lumbar motion preservation with the risk of postoperative coronal imbalance. Factors affecting the choice between LIV at L3 and L4 remain poorly defined. We aimed to identify preoperative factors influencing the selection of L3 versus L4 as the LIV, as well as predictors of immediate and persistent coronal balance (CB) in Lenke 5 and 6 AIS. We hypothesized that preoperative coronal and lumbosacral radiographic parameters were associated with the selection of L3 versus L4 as the LIV in Lenke 5 and 6 AIS. We further hypothesized that these factors were associated with both immediate and persistent postoperative CB. We retrospectively analysed 152 AIS patients with Lenke 5 and 6 curves undergoing posterior spinal fusion, comparing radiographic outcomes between L3 and L4 LIV groups. Multivariable logistic regression and ROC analyses identified independent predictors and thresholds for LIV selection. Linear regression determined predictors of immediate and persistent CB. Fusion extended to L4 in 34.9% of patients (n=53), while 65.1% stopped at L3 (n=99). Independent predictors for L4 selection included younger age, L4 as the lower end vertebra (LEV), and a Type C L3 pedicle–CSVL relationship. Additionally, a side-bending thoracolumbar/lumbar (TL/L) Cobb angle ≥14.5° and an erect L4–CSVL distance ≥18.5 mm were identified as optimal cut-offs for extending fusion. While immediate postoperative CB relied primarily on preoperative CB and main thoracic curve magnitude, long-term coronal balance (≥24 months) was significantly correlated with preoperative CB, erect TL/L Cobb angle, L3 tilt, lumbosacral take-off, L3– and L4–CSVL distances, Risser 5 skeletal maturity, and LEV position at L2. Quantitative thresholds for side-bending TL/L Cobb angle (≥14.5°) and erect L4–CSVL distance (≥18.5 mm), along with a Type C L3 pedicle–CSVL relationship and L4 LEV, serve as objective references for selecting L4 as the LIV.