Are mortgages 5-10 substantialy harder than 1-4?

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I have two properties now and I am planning to ramp up to about eight over the next three years. I was planning to have about 70% equity, 30% loan on each. I've recently read that after your fourth mortgage it is much more difficult to refinance. If this is the case, I'd rather pull more equity out of the first four. Does anyone have any experience trying to get loans 5 through 10?