As far as I'm aware the Mugen battery is the best available at 3200mah. The Anker one being a better value for money choic3 at slightly higher than stock.
I have no way of actually testing this though, but there's no way the OP's battery is anywhere near 4000mah.
On a similar note, does anyone know of any tools to accurately measure these battery capacities?
Current Devices: Samsung Galaxy Note II, LG Nexus 4
Previous Devices: HTC Wizard, HTC Kaiser, HTC Touch Pro 2, HTC Desire Z, iPhone 3G, HTC Mozart, HTC EVO 3D, HTC Trophy, iPhone 4, HTC One S, iPhone 4S