Question

Why is acetiic acid called a weak acid though there are 4
'h'atoms in the molecule

Answer 1

The formula for acetic acid is $CH_{3}COOH$
As we know that acetic acid is a carboxylic acid, $RCOOH$ group
If we look closely, $CH_{3}$ is a carbon chain in which Hydrogen are bonded with Carbon. During reactions the carbon chain or R doesn't participate in the reaction. So, the other Hydrogen bonded to Oxygen is what he's got, which the acid gives up while dissociation.

Answer 2

Acetic Acid : CH3COOH
It's a general misconception that greater the no. of 'H' atoms in an acid , stronger is the acid. It's not like that. Strength of an acid is no. of 'H' ions it dissociate into upon dissolving in water. In the case of acetic acid , it dissociate to give single 'H' ion and that also not complete dissociation. Hence , it is a weak acid.