How to determine the market rent level
You could simply look at other similar rentals in your area. The problem with that method is that you are presuming the other owners or managers are charging the correct rent.
You could target the types of tenants you are seeking by median household income as stated in the detailed demographic information for your property. For example, your screening policy might simply be that a tenant has to have 3 times the rent in monthly income. That is a fast and reasonable method of qualifying tenants provided you have set the rent correctly in the first place.
I have found that detailed demographic information works the best to establish market rents. If you are purchasing a value added property (fixer) or building new, it is essential that you have the data so you do not, as they say, break your pick! Contact Me and I will forward you valuable demographic information about your property and explain how you use it. At no charge to you, totally FREE!
For a moment, let's say I am correct and the demographics suggest your rents are well below market. After a little more research, by you, you acknowledge your rents are $100 per month low and you have 4 units. You may not even be charging pet rents or billing back water and sewer! Are you willing to continue to leave that kind of money on the table? DON'T DO IT, your property is an investment!
I am a landlord myself and know all about the job (it's work!) , subsidizing rents (usually because I got lazy!) and unexpected repairs. So Contact Me and I will tell what you need to know.