I'm on H1b and my wife and son are with me in USA on H4 and they do not earn any income.

1) In order to get tax benefit should my wife have to get a ITIN?
2) I mentioned them in my W4 as dependents, that's not enough to get tax benefit?
3) Should we have to file tax jointly or I should file separately in order to get less tax?
4) For filing jointly should my have to get ITIN?

1) Yes.

2) No; the entry on W-4 just determines how much tax is withheld from your salary.

3) File jointly.

4) Both your wife and son will need to submit Forms W-7 to get the ITINs. ITINs are required to be included on the annual tax return.