hi, can anyone tell me why 3nf/bcnf is a suitable stage to conclude normalisation process, as opposed to 4th and 5th??

4th and 5th deal with issues that simply don't arise in 99% of tables. Of course, when those issues do arise, further normalisation should be considered. So if you only normalise to 3NF, chances are all your tables are good; whereas if you stopped at 1NF or 2NF they almost certainly would not be.