Four grains of dust in as many days? I think “spewing” might be a bit of an overstatement.
That is 4 that Rosetta has collected from between 506 and 111 miles away. This is a short period comet so I suspect it ejects relatively little compared to long period comets.
Interplanetary dust grains are a few micrometers in size, and mass a few nanograms at best. Normally there would be no expectation of dust grains detected in the small volume of interplanetary space surrounding the comet as the probe travels near. If you aggregate all of the material normally present in that volume you probably would end up with a quantity too small to see with the naked eye. Four grains is a thick fog of dust by comparison.
I suspect the dust detector is an impact detector. That implies the dust has some velocity, so substitute "spewing geyser" for "fog".