Before you make something in Sketchup i suggest you use a 2d mapping tool like [http://discon.googlepages.com/mapper.html this one] first
* Make sure the the measures in the [http://img360.imageshack.us/img360/5919/guidesv6.jpg lower right box ] are in metresmeters,if not go to preferences and set it to metric meters 3D in template tab.
* Now select make a square of 4x4 meters. Just start drawing a square and type 4,4 then hit enter.
* Copy the squares to make a grid