general question

Hi everyone,
I was wondering if someone could help me please. I am learning slowly about trading as i go.. and was wondering.. if i have money invested in stocks - can buy more without putting down a deposit - through comsec, it takes four days to take out the money, can i buy and sell within the four days.. effectively not having any money in the bank ???
hope this makes sense..
ok try again, i am trying to say.. can i buy stock and sell them within four days without any money?? they cant take it out after i have sold it right??
has anyone done this ??
any knowledge would be greatly appreciated
cheers
kylie