What weed are you trying to put the material over. Some weeds will grow through feet concrete.

Anything you put on top of plants growing in your soil if covered by anything that will exclude the light those plants need to grow will cause them to die. So 4 inches of gravel or "topsoil" would do that to the plants growing there now. But, any seeds in that gravel or "topsoil", or that get blown in or dropped as the birds fly over, could germinate and grow. That is why some people think that mulches do not keep "weeds" from growing, because new seeds germinate and grow in that mulch.

Some grass weeds such as bamboos and Johnson grass laugh at 6 inches of any thing. So do some of the dock. This is why I asked what type of weed.

Kimmsr is correct that may people see the weeds that grow on top and think they are the original weeds but truely some weeds and weedy plants can grow it seems through anything.

Hey, maifleur, let's add Bermuda grass to the list of "laughers." I have also seen the terrible ground cover Liriope spicata come through a heavy layer of gravel OR soil.
Hortster

I went to Joy Creek Nursery several years ago and saw the power of bamboo. They had a wonderful stand of the big stuff. The shoots were coming up and we were shown how to kick the top to stop growth. The person looked accross the torn up part of the road and metioned that that particular type did go under the road to the other side. When asked if it caused the road problems he smiled and shook his head. You could see holes in the pavement that were the size of the bamboo. After that I do not have any doubts about it growing through any thing.