My personal thoughts...
The market is really high right now(not a time to buy unless you find a gem).
Remember REI is a slow game! REI is a game that grows exponentially overtime. Cash flow is king in REI. The longer the loan term and lower the payment. The lower the expenses are the better the cashflow.
Will refi 15yr and 4.25%, lower your current payments? Have you thought about pulling out equity? If so, is it enough to buy a second 4 plex?
What is your goal?
Slow and safe - pay it off or sell;
200-250k equity and 2500 a month cash flow(when paid off) sounds like a great start. With 2500 a month, that's a new rental house a year. Would be better in current market unless you already have a plan for the cash.
Fast and riskier- refi (take equity out); buy another 4 plex in the area.
People follow one of those 2 plans in buy and hold. The people who got hurt in the crash of 08' were the fast growth people. However they over leveraged themselves.