HVAC units fluctuate in price during different times of the year.

If you were positive cash flow, you would just bank the cash, build up your capex/contingency fund and ignore the market fluctuations. 

Sure you have fluctuation in currency (can go both ways) but investing in real estate overseas has similar risks, and if you are not a citizen your have very little protection, unless of course you are a foreigner investing in the U.S.!

But missing every time just before major market fluctuations?

While real estate can provide higher returns, it is subject to market fluctuations and requires active management.

I also figured that I would include in the lease that the rental price will be reevaluated every X months in case the utilities fluctuate dramatically.  

@Jeff Petsche there’s a federal base in Lawton which people come and go that increases vacancy rates to fluctuate.

I would say these area is the classic hotspot that doesn't fluctuate much because of tech economy.however if the O/P wants to invest in silicon valley area....I would say back in the 90s, "most" techie able to afford home in area like mountain view, palo alto.

Businesses come and go in any industry, and cities fluctuate with them.

If you have to choose between paying more to the lender each month or spending the money, then the 4% return is much better than 0%.The fluctuation in the value of the property has nothing to do with paying your loan earlier.If you keep the money and don’t give it to the bank (lender), you can get over bad times much easier.