Will the current owner be moving out of the unit they're currently occupying? If so, you can run the numbers with the market rent that you'd get upon re-renting. Personally, I do a year-by-year five-year calculation on properties that I consider. That gives me a better idea of what improvements in profitability I can expect in the medium term.
I also agree with @Evan Polaski that you should keep the capex at least at 10%, and maybe higher, depending on the condition and year of construction. It's always better to err on the side of caution as far as building your reserves goes.
Concerning property management, I don't recommend just cutting that out from your expenses. Your time is worth something. Plus, you won't necessarily want to be self-managing forever. A way to possibly cut it down it to find out how much actual property managers charge in your market. You may be able to get away with 7-8%, though with just four units, seems unlikely, and that it would be 10%+. I've even heard of property managers not taking on just a single small multifamily because it's just not worth it, or having a minimum per-unit price. I'd suggest doing a year-by-year calculation, and only allowing a lower property management expense for a limited amount of time (ex: first 3 years). Maybe you'll see that in your calculation, with increasing rents, you'll be able to budget the full 10% property management after a few years.