Here is the professional, SEO-optimized English product description for your Infinix S4 (BL-39KX) replacement battery:

Infinix S4 Replacement Battery (BL-39KX) – 100% Premium Quality

Is your Infinix S4 draining out too quickly, stuck at low percentages, or shutting down unexpectedly mid-day? Bring back its original factory endurance and snappy backup with our premium-grade BL-39KX replacement battery. Manufactured to align with exact engineering standards, this high-quality battery provides the stable and efficient energy supply your smartphone needs for everyday multitasking, calling, and social browsing without sudden power cuts.

Key Features:

• Flawless Physical Fit: Tailored precisely for the Infinix S4 (Battery Model: BL-39KX / Model: X626, X626B, X610B), ensuring an exact structural match and secure connector alignment with your phone's internal layout.

• True 4000mAh Power Capacity: Packs a full standard 4000mAh capacity to completely restore your device's long screen-on time and multi-day standby performance.

• Advanced Smart IC Protection: Equipped with an integrated intelligent safety microchip that actively shields your smartphone from over-charging, over-discharging, unexpected overheating, and sudden short-circuits.

• High-Density Li-ion Polymer Cells: Built using advanced Lithium-ion polymer technology for stable voltage output, entirely eliminating erratic percentage jumps or unexpected countdown drops.