No, it is entirely not suitable for a small child.
If a child of 3-4 month old is irritable I would say its reasonably normal, is the child eating enough, could be colic.I find it inappropiate any Doctor would prescribe any drug for a baby ...I would change my doctor if I were you .
Risperdal is for adults with schizophrenia.
I don't think any doctor would prescribe it for a cranky infant.