If seeking conventional financing, you cannot do a cash out refinance based on ARV until 12 months of ownership.

Conventional loans are an option if you have good credit, but they can be harder to get for flips.

Regular Conventional? 

If you don't sell the 1st lien off, its a lot easier for the borrower to get refinanced out into a conventional loan in 24 months at only a 80% or less LTV (depending on appreciation and market values).I think we can agree on one thing though....no matter which route you go, you have to treat it like a business.  

These loans often have more straightforward qualification requirements for investors with multiple properties but may have slightly higher rates.Cash-Out Refinance on Your Single-Family Rentals: If each has around $70k in equity, a cash-out refinance could provide some capital for the down payment, renovation costs, or even help you buy the 4-plex outright.FHA 203(k) or Conventional rehab loan: If you’re planning to occupy one of the units as a primary, FHA’s 203(k) loan or a conventional renovation loan could help with purchase and rehab costs at lower interest ratesPortfolio Loan: Some local banks may offer portfolio loans for investors with multiple properties.

The numbers worked with a conventional loan, as a long term rental.

LTRs basically do not cash flow at current prices with conventional rates. 

Consider FHA loans, conventional loans, and alternative financing options.

Aiming for conventional and not a DSCR loan due to costs and the higher down payment requirement for the latter.With that said, I'm curious to hear your take on what the most cost-effective & efficient approach would be to source a down payment.

With a rate for a first mortgage today in the 6-7% range something like this probably is going to be close to or in the double digits.