Knowing your long-term plans can help you build a long-term financing strategy so that you are not just worried about purchase number 4, but also purchases 5 - 20+ (if you plan to continue to invest).I'm an investor and licensed mortgage broker.

I would buy a deal that at least minimally cashflows (they exist) and then plan to refi in the 18-36 month window.

Quote from @Sami Gren: I start by making the original tenant fill out a form requesting to add a new renter, and they pay me a $50 processing fee (I charge a $50 fee any time I make changes to an existing lease). 

Quote from @Sean Gallagher: If there is an existing lease, you must honor it.

For example, you might choose to take a blanket mortgage covering all properties, BUT perhaps the market softens and later you wish to sell a property.

This allows you to access funds while keeping your existing mortgages intact.Second Mortgage: Explore lenders who offer second-position loans on investment properties, though rates will be higher.Cash-Out HELOC : While traditional banks often restrict HELOCs on non-owner-occupied properties, some portfolio or private lenders may offer HELOCs for investors.With $15-20K in liquid funds, look for deals where you can negotiate terms:Seller Financing: Negotiate lower down payments or interest-only periods.Subject-To Financing: Assume the seller’s existing mortgage while covering the down payment.Lease-to-Own: Lock in the purchase price while using rental income to build equity.

That's three months of mortgage and utilities, the cost of flooring, and the cost of painting.

I read that one of the requirements for getting your second FHA loan is having a change in family size. Has anyone gotten a 2nd FHA loan this way or know of someone who has?? I'd really appreciate any and all helpful ...

I do not understand your remark as related to the DSCR loan.  2 ADUs are not allowed by f/f conventional financing but is not always enforced.Fortunately if you stay with the current tenant without a refi, by the time you pay principle it is likely the rent will have increased enough to absorb the increase mortgage payment. 

It also includes managing your existing investment well to ensure you aren't bleeding money due to bad tenants, excess maintenance, etc.