Educate yourself on the local market, network with other investors and agents, and always have a solid plan in place before making any investments. 

For BRRRRs, you'll need to have a solid team.

You'll spend a lot of time (sounds like you may have already) spinning your wheels in trying to find the right lender, whereas a solid broker can pair you up with the right fit right away and get your project moving. 

Grass roots marketing/networking/free social media 2.

Not sure what the layout looks like or location - but in a solid location, which I'm assuming since it's an STR, you should be able to in the ball north of $800-1000 per room and even more so for the rooms with private baths.

It sounds like you’re on a solid path!

Given your experience and strong financials, you have some solid options:Commercial or Portfolio Loan: These are tailored for investors and can help you avoid personal guarantees, especially since you have an LLC.BRRRR Loan (Buy, Rehab, Rent, Refinance, Repeat): Some lenders offer BRRRR-specific loans with funding for purchase and rehab, allowing you to refinance based on the new ARV once stabilized.HELOC or Cash-Out Refi on Current Rentals: Tap into the equity on your single-family rentals to secure funding for the 4-plex purchase and rehab without high origination fees.Each has pros depending on your cash flow goals and timeline.

Aiming for each part of the business to eventually fund itself is a solid plan, and a real estate CPA or attorney could help fine-tune everything for taxes and structure.

I watch a lot of David Greene's YT videos and have messaged him directly and he gave me solid advice - if I'm going to buy outside of California, to go for appreciation states and look at the asset. 

The solid slap of a sheaf of used bills in my hot little hand always gives me a little tingle.