Oh what I understand about it, it's a competition where you have to make the best item you can using 1 standard length of 2x4.
I think it originated in America.
I suppose It could always be altered to use a 2x4 length of Oak, Ash, Cherry etc
I found this on another site
"The 2x4 Challenge is all about testing your creativity and imagination. What can you do with a single piece of 8 foot length 2" x 4" pine? The world is your lobster, turn this bland material into something beautiful!
The wood can be any species under the softwood pine banner. The dimensions of what is commonly called a 2x4 can differ from retailer to retailer. It also depends on whether you buy a PAR (Planed All Round) board or Rough Sawn. Furthermore converting imperial to metric and visa versa will differ depending on how it's been rounded up or down to the nearest equivalent measurement. With all this in mind we ask you to use common sense and honesty in choosing your timber. As an example a planed 2x4 in somewhere like B&Q will be 44mm x 96mm. The length should be no longer than 8ft or 2400mm (or your retailer's equivalent).
You may use any method of fixings you chose (Biscuits, Dominos, Glue, Screw, etc) and are free to use any wood finish you chose (Paint, Stain, Dye, Oil, etc). Hardware such as catches, latches, pulls, hinges are all acceptable.
Other building materials, such as metal, plastic, stone, etc, may not be used at all. There are no restrictions on tools/machinery/techniques used. Woodworkers, Turners, Scrollers and Carvers are all welcome."