Should we pay off or refi and buy another property?

[Julesflex]

I have a rental property on a 15 yr mortgage with only 5 yrs left. Should I refi and take money out to buy another property or pay off the mortgage as soon as possible. Rate is 5.25%, mortgage is $1800 pm, it rents for $2500 pm. We are in our 40's, live in CA and have 4 rental properties. I'm torn between piece of mind having a house paid off, and the tax advantages of continuing a mortgage and buying another property while the market is at a low.