Can I pick your brains?

I'm currently bored out of my skull, and I pick up a new edition of New Scientist.

There is this question thing which appears simple at first sight, but is quite difficult.

Just wondering if anyone can figure this one out.
I'm just a bit brain dead here... spent about an hour trying a bunch of calculations but to no avail.
If you got some spare brain cells and time, lets see if any of you could make anything of this.

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A pipe manufacturer ships pipes of three different radii in the same square section box.
The pipes just touch each other and the box as shown above.

If the middle sized pipes have the radii of 4 centimetres,
what is the radii of the smallest pipes?
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Here's proof that I didn't make the question up:

Source: NewScientist - Issue 24th July 2004 Bottom of Page 51