A microprobe was a type of small probe carried aboard Federation starships that was used in cases where conventional probes were too large to be effective. One such type of microprobe had the designation of "class 4".
Later that year, Voyager launched a microprobe into a micro-wormhole to trace its subspace bearing, but the probe became lodged in a gravitational eddy inside. Harry Kim subsequently modified the communication system so the probe could act as a subspace relay station, allowing Voyager to make contact with the Talvath on the other side. The probe was eventually crushed by the wormhole's collapse. (VOY: "Eye of the Needle")