Mr Dinesh,
I wanted to suggest a different course of action.
The employees need to be called one by one and enquiry made about their chats and views about the company.
The group is them four only and not whole company.
Have they been counselled for non performance in writing or even verbally?
Just because they spoke anti CEO etc should not lead to hurried termination and legal consequences.
Give them an option to quit voluntarily, if they quit, no legal hassles will come up.Smoothly ease them out of the company.
Private criticism of bosses is very common and emotional reaction is avoidable. The whole sequence of criticism is a private chat and not made on public platform or for all employees.Basically they need to be strictly counselled and given a chance to reform and contribute.There may be a reason for unhappiness and only they have got caught, since they used Office SIM /Phone.Precipitate action will not be in tune with needs for natural justice.