A bank can offer you any kind of mortgage (rate, amortization, balloon payment, adjustable rate after 3, 5 or 7 yrs etc.). They usually refer to the loans for which they make up their own rules as 'commercial', to set them apart from the 'conventional' mortgages, for which the rules are set by FannieMae/FreddieMac.
The rules you are citing (4 unit or less, amortization 30 yrs, also 15 yrs) are FannieMae/FreedieMac rules. If a bank wants to enjoy the guarantees that FannieMae/FreddieMac provide, the banks need to follow their rules when issuing mortgages. One of their rules is that a person can only hold up to 10 mortgages. This means that you, the landlord, can finance up to 10 single-fam homes using the extremely advantageous conditions of these government-backed 'conventional' loans. Run the numbers to compare cash flow from a conventional 30-yr mortgage and a commercial mortgage from you local bank.
You should try to find a broker or bank that is willing to sell you conventional mortgages up to the 10-mortgage limit, simply because the cash flow is going to be so much better. Call a few brokers in your area.