Section 44ad of income tax

ITR 4S cannot be used only by professsionals. It does not necessarily exclude all service providers. Works contract service provider can file 4S if otherwise qualified.

About private tuition fees, I am not sure if you should account it as business income or other sources...

Coming to private tuition class services, if such services are provided by the individual assessee, then such income will be considered as professional income in which case ITR4S is not applicable. But if several tuitors have been hired by the assessee to run classes, it might be considered as business, in which case ITR4S might be applicable. However, I would await views of others.

But if several tutors are hired by an assessee for teching & assessee himself also teach students in the tuition class, then what ? whether he can file ITR 4S or not ?

And one more thing I just want to clear is : Is it true that all the service providers can not use ITR 4S? only a person doing business can file ITR 4S ?

Whether a particular activity can be classified as `business' or `profession' will depend on the facts and circumstances of each case. The expression "profession" involves the idea of an occupation requiring purely intellectual skill or manual skill controlled by the intellectual skill of the operator, as distinguished from an operation which is substantially the production or sale or arrangement for the production or sale, of commodities. - CIT Vs. Manmohan Das (Deceased) [1966] 59 ITR 699 (SC), CIT v. Ram Kripal Tripathi [1980] 125 ITR 408 (All).

By reading the above definition, this seems to be profession only and hence ITR4S cannot be used. ITR4S can be used for business purpose only. If professional income, then ITR4 needs to be used.

One more thing If a civil contractor gets his contract done by his labourers, then is it a business or a profession ?  

I think that is business. https://incometaxmanagement.com/Pages/Gross-Total-Income/Business-Profession/Which-Professional-Businessmen-have-to-maintain-Bookof-%20Accounts.H T M L