The lighter the better, but, when it gets to lightwight it gets hard to manage. Kinda like cling wrap for leftovers. In my construction work I use a lot of 6mil visqueen for vapor barrier under concrete slabs. Um. now come to think of it, the painters plastic is much lighter than 4mil. I'll have to check that. But I know it's light. And yes 4 mil is not very light. It needs to be somewhat durable to withstand being pierced by a blade of grass anyway.