There is probably only somehwhere in the range of 50 multifamily properties (2-4 units) that exist in the entirety in Arlington. You might see 1 or 2 sell per year. Expect prices on them to exceed $1.3 million.
There are more in DC proper, but not a lot in the entire region in general. In 2017, 218 sold in the entire DC metro area out of roughly 90,000 home sales.