Jeremiah,
With four properties currently mortgaged, you are going to have both of these options taken away from you as you currently know them in the very near future.
You are going to be forced into commercial financing. When that happens, find a lawyer and a CPA familiar with that world and let them help you navigate the waters.
As a strategy, high leverage can be a good thing when you are starting out, but once you reach critical mass, it becomes an albatross around your neck.
If your planning shows you need the income from 40 sfh rentals to support you through retirement you want those paid for as soon as possible after purchase.
With each paid for house you do two things, you improve your financial strength going into any future deal and you reduce the real risk you face when things don't go as planned.
A couple of years ago, we had the start of a major real estate correction. I say the start because it was prevented from fully correcting. The correction will complete, I think it will do it within the next 5 or so years and I have put my money where my mouth is by selling out all interest in real estate investments in the US. The only real estate I currently own are our houses, all 100% paid, and the building in Seattle where the offices of the software company I run is located, again 100% paid for.
Given what I firmly believe is coming, those using leverage need to be keenly aware of its risks and the warning signs indicating the need to reduce that leverage. Timing the market at the micro level is a crap shoot at best.
Now, having said all that, if you are going to continue to buy in this market to hold long term, then for the next couple of years at most, I would buy with 25% down. Then start piling up cash. Once the real correction happens, you can join the real investors like me who will snatch up deals that are considered unimaginable today.
Donald Trump is an arrogant, self aggrandizing, jerk. But, you should definitely follow his lead on this and others like him... PILE UP CASH IN INFLATION PROTECTED EASILY LIQUIDATED ASSETS if you want to have the time of your life as an investor in a few years.