Background: Amcenestrant is an optimized oral selective ER degrader (SERD) that antagonizes and degrades the ER and has demonstrated favorable preliminary safety and antitumor activity as monotherapy and in combination with palbociclib in postmenopausal patients with ER+/HER2− advanced breast cancer, irrespective of baseline (BL) ESR1 mutation status. Here we present the final results of AMEERA-4 (NCT04191382), a Phase 2 preoperative WOO study that evaluated the PD activity of two dose levels of amcenestrant or letrozole using paired biopsies assessed for biomarkers. Methods: Postmenopausal women with operable stage I–III (tumor size ≥ 10 mm by ultrasound) ER+/HER2− breast cancer and Ki67 levels ≥ 15% by local assessment were randomized 1:1:1 to amcenestrant 400 mg once daily (QD), amcenestrant 200 mg QD or letrozole 2.5 mg QD for 14 days before surgery. Antiproliferative activity, as measured centrally by change from BL in Ki67 using paired biopsies before and after a 14-day treatment with 2 different doses of amcenestrant vs letrozole, was the planned primary endpoint. Main secondary endpoints included ER target engagement, assessed through the change in ER expression measured by change from BL in H-Score, and safety. Results: Trial enrollment was voluntarily stopped early, as informative data supporting adjuvant development became available; therefore, no formal statistical comparisons were conducted. Among 105 randomized patients (amcenestrant 400 mg: n = 34, amcenestrant 200 mg: n = 36, letrozole: n = 35), 95 were treated and had available pre- and post-treatment Ki67 per central review (modified ITT population). No major imbalances in BL patient and tumor characteristics were observed. The geometric least squares means (LSM) estimate (95% CI) of Ki67 reduction was 75.9% (67.9, 81.9) for amcenestrant 400 mg, 68.2% (58.4, 75.7) for amcenestrant 200 mg and 77.7% (70.0, 83.4) for letrozole. The LSM estimate (95% CI) of absolute change from BL in ER H-score was −176.7 (−201.4, −152.0) for amcenestrant 400 mg, −202.9 (−226.1, −179.7) for amcenestrant 200 mg and −32.5 (−57.2, −7.7) for letrozole, with median relative changes of −65.3%, −68.3% and −9.5%, respectively. The incidence of treatment-related adverse events (TRAEs) was 21.2% for amcenestrant 400 mg, 22.2% for amcenestrant 200 mg and 25.7% for letrozole. No Grade ≥ 3 TRAEs occurred in any treatment arm. Conclusions: Both doses of amcenestrant demonstrated robust Ki67 reductions, strongly engaged the ER target, and continued to show a favorable safety profile in an early breast cancer population, consistent with previous published reports. Based on PD activity and safety, and emerging results from other ongoing amcenestrant trials, the 200 mg QD dose of amcenestrant was selected for our ongoing study in the adjuvant setting; AMEERA-6 (NCT05128773). Clinical trial information: NCT04191382.