Rough market ranges, and they do move by market and by company, so treat these as what to expect on a quote rather than a price list.
Full management on residential rentals commonly runs 8 to 10 percent of rent collected, with small portfolios often quoted at the top of that or with the per-unit floor @lantern described. Tenant placement is typically 50 to 100 percent of one month's rent. Many managers add a markup on maintenance coordination, often 10 percent of the invoice, and a renewal fee of $100 to $300.
If you keep self-managing, landlord software for four units is cheap. Several platforms are free to the landlord and charge the tenant a small fee for card payments, with ACH free, and paid tiers generally sit in the $15 to $40 a month range at your size. Tenant screening reports run about $30 to $60 per adult applicant, and it's normal for the applicant to pay that as an application fee, though some states cap what you can charge and some require you to refund the unused portion. Bookkeeping help, if you want it, tends to start around $75 to $150 a month for a handful of units.
The cost people forget on small multifamily is local rental registration. Plenty of cities require you to register each rental unit and pass a periodic inspection, with fees per unit and fines for missing the deadline. That's a city-level rule and it varies widely, so check your municipality directly. Four doors means four registrations in most places that do it.