Roy N.
In comparing expenses between MFHs and SFHs, you're assuming that the alternative to buying 1 x four-unit is buying 4x SFHs....I don't think this is a valid assumption. I would offer that the price of a four-unit would probably only get you 2 - 2.5 SFHs in the same neighborhood....the price of a duplex would buy about 1.25 SFHs, etc.
My point is, when comparing expenses in determining whether MFH or SFHs cash flow the best, I don't believe you can say that a 4-unit only has 1 roof (1 driveway, 1 set of walls, etc) while the SFH alternative has 4 of each. In actuality, I only think you'll be able to buy 2 - 2.5 SFHs...and the larger MFH roof, walls, etc. will bump the expense of replacing it beyond a normal SFH replacement.
I said in my earlier post that I think expense difference between a MFH and the SFH alternative is a wash. In my opinion, what makes MFHs more attractive over SFHs is the fractional rent/unit vs. price/unit.
For example, in my area you can buy a 4-unit (2br/1ba each) for about $140K ($35K/unit). You can rent the units for about $575/unit - or a rent/price ratio of about 1.6%
That same $140K will get you 2 comparable SFHs ($65K/unit). You can rent them for about $675/unit - or about 1.2%. Granted, you'll have about $10K left over in this example.
So...long post only say that I agree with everyone - MFHs are a much better cash-flow option than SFHs but, in my opinion, it is not because of a reduced expense burden. At least in my area, the advantage comes because of a favorable rent/price ratio.
Now, of course none of this discussion factors in appreciation over the long term...so for total-return investors SFHs might have an advantage.
Sam