The included batteries are not mean to be run at WOT for more than a few seconds at a time.

As you've discovered, you only do get about 4 minutes or so this way.

The plane can fly for about 8 or so minutes if you keep the throttle down to around 60% - 70% after takeoff, and throttle back whenever possible.

Switch to LiPo batteries and things change completely.

The plane will have more power and much longer run times, if you choose the right set of LiPo's.

You can use the 5S configuration stated in the manual, that is purchasing a 3S 11v pack ( 3200mAh or greater ) with a matching 7.4v 2S pack ( 3200mAh or greater ) as long as both packs have the same capacity... note: both packs must be charged outside the plane and seperately!

Hint: Hobby City has GREAT deals on the batteries:

3S 11v [link=http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=7295&Product_Name=Rhino_ 3700mAh_3S_11.1v_20C_Lipoly_Pack]Click me for 11v[/link]

2S 7.4v [link=http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=7651&Product_Name=Rhino_ 3700mAh_2S1P_20C_Lipoly_Pack]Click me for 7.4v[/link]

Hint purchase 2 of each!

Why?

Match pairs of batteries can be combined in many other useful ways...

e.g.

You can combine the two 7.4v packs to create a single 14.8v 3700 pack capable of powering a larger plane.
You can combine the two 7.4v packs to create a single 7.4v 7400 pack capable of powering a plane for a much longer amount of time.
You can combine the two 11v packs to create a single 22v 3700 pack capable of powering a larger plane.
etc. etc. etc.

At worst you end up with two pairs, one to charge and the other to fly with, that should give you FAR longer flight times if you resist the urge to fly at WOT...

Don't be surprise to see 16+ minute flights out of your plane... on a charge...