A thread over at Mantic on the topic of terrain brought this to light:

25% terrain for a 4x6 table

OR

6 pices of terrain for a 4x6 table

Both of these suggestions were made by rules commity members. I am no math expert but I would guess that 6 pieces works out to roughly 25% of a 4x6. So long story short both should work, just make sure each table has 6 pieces.