Commercial property Insurance?

I recently purchased a 4th house but my home insurance denies further coverage and tells me that I can only insure up to 4 houses and must switch to commercial insurance. Is this normal proceedure? Is there any commercial insurance companies that are good and you recommend? Are they as good as the non commercial when you have a claim? Thanks;

Allstate will write up to 15 landlord policies per policyholder, b4 switching to commercial.

Any type of commercial insurance is expensive, I'd avoid it if possible.