So I am glad you are 100% as a long investment because this will make my point even clearer.
If you pay full cash at 120K and you expect 1200 per month.
Lets say that 1200 per month is pure profit and you have no expenses, not insurance not property taxes, nothing. It will take you 8 1/3 years for you to realize any profit at all on the property.
So taking into account part of that 1200 each month has to go to at least insurance and property taxes your break even time gets extended even further out.
If you use a mortgage you are taking money from a bank to buy a property that the tenants will then pay off the mortgage.
Another way of looking at this:
Lets say all things are equal and you end up holding this property for 30 years and you never have a single expense and you pocket all the cash:
A 30 mortgage at 5% with 30k down will cost you $483 per month for 360 months
so if you pocket the excess each month you will have (1200-483)=717 per month x by 360 months gets you $258,000 for only a 30k investment
now take that number and multiply by 4 because with 30k down you can buy 4 properties and you have $1,032,480
so 120K = $1,032480
Your way will pocket 1200 per month for 360 months = $432,000
so 120k=$432,000 at the end of 30 years
now of course there are extra costs included with the above calculations and thats where owning multiple properties spreads out your risk.
First off if you are buying 4 properties that are going to be vacant all at the same time then you are buying the wrong properties. Usually what happens is you have one vacancy you have 3 other properties to cover the costs of the one vacancy.
Then lets say you have one bad tenant that destroys the place and it takes 2-3 months to get the place in shape to rent it again. Where is the money coming from for that. With one property it is coming out of your pocket. With 4 properties it is coming from the other tenants rents.
A lot of the risk of what your talking about with vacancies is avoided by properly screening your tenants and if your getting 1200k per month your tenants should be decent.
Looking back at your above example of 5 properties costing about 4400 per month then what is your cash flow?
5x1200=$6000 per month. After all expenses are paid in your above example you are pocketing $1600 per month (more than the 1200 you are getting in one month with one property and not even taking taxes and insurance into account). And at the same time you are paying off the properties with tenants rents aka money that is not yours and at the end of 30 years you have 5 properties that are paid off each with a value of 120k.