they are made of two clefts of wood i believe. however if you think you will pick one up and smash away, thats not quite the case, it wouldn't be hard for a batsmen to gain the strength to swing a 4lb bat, but over the long run it would really damage your arms and harm your technique.
colingwood uses it so he can build strength to swing a regular bat faster as prep for ipl games. you would lose so much manouverability from having a four pound bat, it would be really hard to hit shots properly.
i play tennis quite a bit, and i usually play with a 300g racket, but i played with a 420g racket once, after half hour my arm felt so tired, i could barely hit any shots correctly.
if a 4lb bat really was a superior hitting tool than a high 2lb bat, every player in t20 would be using heavier bats. The key is light bats, with very good ping and sweet spot.