Pok did a capacity test on a trustfire flame 5000 protected cell and it came up just shy of the 4000 mark (3946 mah @ 1A discharge).
thread here....
https://budgetlightforum.com/t/-/7999
Also... DX correctly specifies this on their product description....
"- Capacity: 5000mAh (actual 4000mAh)"
http://www.dealextreme.com/p/trustfire-rechargeable-3-7v-5000mah-26650-li-ion-batteries-pair-107232