Sometimes clients do not understand our questions properly. I would check their CRA account and make sure that they really did not file anything. 

Then check with their year-end accountant. 

Also make sure to check your mandate with them. You do not want to work on the last four years if your mandate is only for current fiscal year.

I forgot to say best of luck :)

Thanks

Prachi 

I confirmed it no taxes have been filed, and there are transactions that need to be entered for prior years.

And what is your mandate?

For the whole time, 4 years

I would treat it as a regular bookkeeping job then. Also, try to get a CPA involved.

Thanks

Prachi