I read all the posts- good advice here. I've been renting homes since 2004 full time. My wife tells me I should write a book of all things that can go wrong. I will not scare you - don't worry! If you want to know things to watch for, while having a razor thin margin ( after all cap-ex, vacancy., etc. are removed):
1. Mandatory: If you are buying 4 properties at one time, do not purchase without an inspection, since they are occupied. If the homes were vacant, I most likely would not need an inspection, since I know what to look for. Here are major items overlooked by lots of new investors that will eat up any profits for the year:
Termites: Look for dirt "trails" in the lower portion of each house. Termites are eaten by ants - are there any ant trails? Any ant mounds near the base of the home? Is there any evidence of termites -inside/outside? Any cement previously drilled through where a termite company injected their chemical and patched the concrete? Any termite plastic traps (with wood bait inside) around the perimeter of the home- that look "old"?
HVAC: Do they exist?...or only window AC? Does each house have both types? That screams that the main units do not work. Guess who's responsible for the $4K+ bill when you either sell them retail or try and get tenants?
Water heater: not as big a deal as the above...but they tend to last 10-15 years max. They will usually start leaking before stop working. Maybe the burner needs replacing - you'll know when a tenant tells you no hot water. Figure on $1100 each to replace, $300-400 if the burner needs replacing and you call a handyman/plumber.
Roof: Are you sure the roof is in good shape right now? Just because it's not leaking doesn't mean there isn't hail or wind damage to the shingles (happens a lot here in GA- I've replaced more roofs than I care to remember). A roof inspection may save you $7K per house.
Mold/radon: A thorough inspection inside will reveal a lot. I've found mold behind the baseboards - due to the tenant not running the AC in the summer months. AC acts like a dehumidifier. Humidity built up behind each of the baseboards - found out when replacing the flooring and I pulled off a board- all green mold behind it. Had to remove every one and replace. Radon- can't smell it. You're legally responsible if any exists and tenant gets sick. Also- must prove when selling retail that it is below gov't standards.
The above are most of the larger items to inspect before buying. Remember- if you plan on operating at $90/door per month, that's $4300/year with four houses, gross income- not net. Your cap-ex of 7% will help - but may not cover a large expense...depends on when something needs replacing. Also-any of the above can wipe out any profits for the next year or more. The good news...usually only one thing happens at a time, and not to all properties. Just be aware that this stuff always exists.
For the record, if you show $356/month to cover 4 individual houses - I would not consider this. if you are able to lower the price of the purchase - it may make it more palatable...maybe. Good luck!