battery help

I would go for a 4,000mAh 3s

This one? -
If so, that's a Ben Buckle, not a Multiplex.
Can't find one made by Multiplex.

Quorneng put - but also the prop pitch needs to be sufficient - - blah- blah - - to the static thrust.
-
I don't follow that last bit myself!
If the prop pitch and diameter is such that the motor is not overloaded - then the motor rpm's cannot increase to higher than it's maximum designed per volt (the Kv number).
An under-loaded motor still will not spin any faster, but will not produce enough thrust from the prop to fly the aircraft.
Even if you take the prop off then give the motor full power, it will not spin any faster than it's designed speed - obviously with zero thrust and not much current load!

An over-loaded motor will draw more and more amps/watts as it attempts to turn at it's designed rpm's, and may increase it's rpm's up to or near it's designed max as the loading reduces in flight. (which is why a 25% 'headroom' is advised for power rating of speedo's, etc).

The Watts drawn will drop a little as the prop unloads as it reaches it's forward motion/pitch speed, but the motor will not go any faster.

Where things 'go wrong' in this 'model' is where you have an extremely high-pitched prop - such as on a racing model - at full rpm's, but with a static model - ie just before launching it - the prop will almost certainly be cavitating, effectively stalled, and not producing much thrust (but lots of noise) and drawing LOTS of current as the motor tries to reach it's Kv.
Then as the aircraft is launched, the prop starts to 'bite', the airspeed of the whole increases, and the prop grips (if you can grip a gas!) giving even more airspeed. The motor rpm's might even reduce as the model accelerates, then increase again, back to max Kv as the speed through the air does, and the prop unloads.

Ah - Multiplex Magister - this one -
looks a bit like a Cessna.

Model: Multiplex Magister
Wingspan: 64.2in
Fuselage Length: 46in
Flying Weight: Glow-Powered 67oz
Flying Weight: Electric 84oz

Includes:

Hitec Laser 4 Radio
Hitec HFS-04MG Receiver
Permax 680 Motor
APC 12x8 e-Propeller and 3:1 Gearbox
3 Mini HD Servos
M32 Speed Control
1900mAh Motor Battery
Multiplex 5008 DC NiCd/NiMH charger


Note Permax 680 is a brushed motor, and the model is geared down 3 - 1 to turn the big prop.
The 84 oz flying weight is probably based on Nicads, which WERE heavy!
Elapor is not the lightest of materials, ether.