I am not to discuss rent, pets, deposit or other lease related terms with people who are looking to re-rent my unit.

4 pets?  

My big concern is that this applicant has a service dog and I never allow pets.

Pet deposit was received for pet.

You'll have to decide what criteria is more important to you to use as a "tie-breaker" criteria - higher income, higher credit score, no pets, etc. 

Vacancy, turnover, and not enforcing your lease (late fees, evicting, documentation, pets, inspections, violations, and more).

They don't think about pets, their affect on common areas shared with other tenants, and the landlord's responsibility to provide safe common areas. 

You may want to consider testing the carpet for pet urine.

I would be weary of pets as well but that is up to your discretion.

families & couples ( with a pet) .. the way the units are laid out.. can be used as single big 6 bed house as well.. but what I dont know is how many would in the future want to share spaces like a backyard or a common area and yet have their own privacy in their own units