Separate HVAC systems are a good idea too—makes things easier for tenants and utility billing.I’d also recommend talking to a contractor or local utility company about what’s needed for separate power meters and mailboxes.

What do you recommend for hot water leak alert system?   

Quote from @Jim Bryant: This is why it's important to understand how the legal system works before investing in an area.

With the income issue, there are screening systems available that verify income by having applicants securely log in to their payroll provider, which then shares their gross and net income with you.

ONe its not guaranteed your renters can and do get kicked off the system for violating the policies.

If you are still with CR of Maryland they will transition you to their management system. 

If one or more units are vacant, the remaining units often don’t provide enough cash flow to cover expenses, making it harder to meet operating costs.Maintenance costs: A fourplex comes with four times the appliances, plumbing, HVAC systems, and other components to maintain, leading to significantly higher repair and maintenance expenses than single-family homes.If you want to see the detailed calculation, read this BP blog - More Units Doesn’t Mean More Money—Why a Single-Family Home Can Beat a Fourplex.Resale value: Multi-family properties have a limited buyer pool—mainly investors—who base their offers on CAP rates.

This is why I stress the importance of knowing the laws and the strength of the legal system before investing in a particular market.

Create systems and processes right off the bat.

There can also be chemicals in the treated lumber that are harmful to the environment.Invest in a good screen or built in door system.