Question

if valency of ammonia is 3 then why isn't that valency of ammonium 4​

Answer 1

The valency of NH4 is almost always -1

Answer 2

Ammonia is a neutral molecule.

it has the ability to combine with other atoms/molecules via ionic or covalent bonding. i.e. it can react with HCl to give ammonium chloride - NH4Cl - the ammonia, in effect accepts a hydrogen(+) ion from the HCl. this gives an ammonium ion (NH4+) which has a valency of +1 (The Cl ion left over has a valency of -1)

the ammonium ion will almost always have a valency of +1