What's up Angel! Nothing wrong with being a bit new - there are options out there for all stages of the game.
When you say your house hack phase has passed, are you saying you DON'T plan to occupy a unit of the 4-plex? It'll strictly be non-owner occupied? Lending options will look different depending on your answer. As far as pulling equity from your duplex - totally doable! You've got 3 options:
1. Cash Out Refinance
This will allow you to pull a portion of your equity and will replace your current mortgage (assuming there is one). If it's free and clear, that's OK too. If you ARE occupying, you'll need to be able to qualify using your income, employment history and DTI. If you aren't, there are other options that don't require those. Instead, the income (or potential income) of the property would be used for qualification. FICO is a factor in both scenarios. This option will have the lowest interest rate.
2. HELOC
This takes either first or second position (depends on whether you have a mortgage) and is a variable rate line of credit. Interest rate will fluctuate. Typically, you'll have something like a 10 year draw period with interest-only payments, and then the balance is amortized over 20 years. Most HELOCs will require that the property is titled to you, not an entity (like an LLC) and require using your income, employment history and DTI to qualify.
3. HELOAN
This takes second position and is a closed end loan, meaning it's given as a lump sum similarly to a "regular" mortgage. Qualification can be either of the methods I mentioned with option #1.
Hopefully that helps! Happy to chat if you have any questions :)