This depends on whether the business was sold and it is now an entirely different entity, or whether the shares were sold and the new owner carries on business under the same company name. If it is an outright sale, then yes, you have to process T4's for the company who sold. If the sale was for shares only, you do not have to process T4's as there will be no break in employment for the employees; they will continue to be employed by the same entity.