Accessing a movieClip on the stage from within another movieClip

Hi, after many hours searching through many forums and posts im still not finding the answer to what should be pretty simple to achive. If anyone has an idea please do tell me.

Problem is this:-

I have two movieClip’s on a stage (on the main timeline). Called ‘box4_mc’ and ‘fadeSquare_mc’. Inside ‘box4_mc’ on its timeline (but only on frame 54) is a button called ‘close4_btn’. When I click on ‘close4_btn’ I want ‘fadeSquare_mc.play();’ to work. The actionscript code has to go on the ‘box4_mc’ timeline because the button doesn’t exist to the main timelime. I have attached a .fla file zipped up as an example. I have tried the following:

 
this.parent.fadeSquare_mc.play();

and also

 
this.fadeSquare_mc.play();

the first gives an error 1119: Access of possibly undefined property fadeSquare_mc through a reference with static type flash.displayDisplayObjectContainer.

the 2nd gives
TypeError: Error #1010: A term is undefined and has no properties.
at boxes4a_fla::box4_12/closeBox4()

im at a loss what to do.

Thanks in advance.