That depends on the 40 sized motor and what you want to do with it.  Post what it is (40FP?  40LA?  K&B?), what you're doing (stunt?), whether it's been modified, and maybe even what plane you have it on (weight and size matter).  Then the folks who actually know what they're talking about (not me!!) will jump in and help you out. 

If it happens to be a Tower 40 then this thread should be of help: http://stunthanger.com/smf/index.php?topic=30920.0.

If you read enough posts here and do even a bit of your own experimentation, you'll find that each individual plane has its own best prop, and your flying style affects your choice as well.

What motor?  Different motors take different props, even if they're the same displacement.  If just want to fly in circles and if you have the instructions for the motor, start with that.  Go with the biggest lowest-pitch prop they recommend, unless your 4-1/2 pounds of airplane also sports a teeny wing.

4.5 pounds seems like a lot of plane for a 40.  What is it?

p-51

then why dose a .40 evalution motor work on a hanger9 mustang very well?

If all you're going to do is fly level laps with maybe climbs and dives, then even a 7-8 pound airplane with a 40 may work OK on control line.  That's assuming that it doesn't pull you off your feet.

But most of the guys here are thinking "stunt", and control line stunt is far more akin to RC 3D in it's airframe and power plant requirements than it is to RC warbirds.

Basically with stunt you're routinely carving figures in the sky whose diameter is about 12 times the airplane wingspan, some of those maneuvers are required to have corners that are more like 2-4 times the airplane wingspan, and some of those maneuvers are required to be done straight overhead, with the airplane basically being held up by centrifugal force that's maintained by the engine pulling the plane straight up.

And for stunt you're locked into whatever the engine does by itself -- consequently, engines are adjusted for steady runs or possibly for runs that give you more power when you need it.  This means that the peak power available is considerably less than the power available from the same engine when it is set up for RC.

If you are flying a 70 ounce plane then you had better go to .018 lines and hold on tight. If you are flying one of the cardboard planes from Flying models and you did the controles like the plans show I do not think the plane will last long. He shows his controles mounted on a piece of plywood between two cardboard sides. If the plane is not flying level it is tail heavy as some one has already mentioned in another thread. If the plane has a large nose try a 12/5
 Also try typing more than one sentence if you want anyone to take you seriously.
Ed