@Brendan Connolly It might help to let BP members know where the properties are located and where you are looking to invest.
1 - Cash out refinance, HELOC (Never done myself, but I have heard it can be difficult to get), or just get financing. I get the impression that the 4-plex is doing well so I personally wouldn't sell it.
2 - Whether you go with LLC or not, is a question that only you can really answer. Do you own a lot of other assets? If you do, how comfortable do you feel having them exposed? You should consult with your lawyer and make that decision based on their advice and your tolerance for risk.
3 - I am not a huge fan of paying off properties especially if I just have one. In your case, you would be tying up most of your capital that could be used to expand and diversity your portfolio. If something happens to that 4plex, all that capital is at risk.On the flip side, you have a property free and clear with much higher cashflow. You will likely be able to weather any downturns if you keep the units rented. So again, this comes down to your risk tolerance and your goals.
Hope this helps. Feel free to message me if you got more questions.