Molecular dynamics is exploited to examine mechanical characteristics (stress-strain dependence, tensile strength, and maximum elastic strain) of graphene sheets containing 5-5- 5-9 defects. Each such a defect represents a tetravacancy (localized group of four vacancies) associated with .pentagon-pentagon-pentagon-enneagon. atomic configuration in hexagonal crystal lattice of graphene. We revealed that the tensile strength of graphene sheets dramatically degrades (by 5-7 times) due to the presence of 5-5-5-9 defects in graphene, as compared to the tensile strength of defect-free graphene. Also, results of our computer model indicate that the mechanical characteristics of graphene sheets containing 5-5-5-9 defects are highly sensitive to geometry of their mechanical loading.