In the third measure shown, the A has two stems (one ascending, one descending), a circle drawn above it, and a numeral 4 written below it. I know that a double stem like this can mean a divisi part; and an open circle can either mean a harmonic or an open string. But I can't figure out what the 4 is supposed to represent. (I'm a brass player, myself, so I'm not familiar with a lot of string-specific notation.)
What, precisely, are the second violins meant to do when they play this note?