A 4000mAh 18650?  Yep it's real.

This is the highest capacity 18650 battery in existence.

Vapcell direct. These batteries are direct from Vapcell.

Priced per battery, not per pair.

Vapcell N40 18650 10A 4000mah Specifications:

• Size: 18650
• Approximate Diameter: 18.3mm +/- 0.2mm
• Approximate Length: 65.0mm +/- 0.3mm
• Brand: Vapcell
• Manufacturer Rated Max Continuous Discharge Current: 10A
• Max Discharge Current: 12.5A (with 60 °C temperature cut)
• Manufacturer Rated Typical Capacity: 4000mah
• Manufacturer Rated Minimum Capacity: 3850mah
• Nominal Voltage: 3.6V
• Peak Voltage: 4.2V
• Protected: NO
• Style: Flat Top
• Color: Red and White
• Origin: China

Manufacturer Discharge Graphs: