A class 4 probe investigates a subspace rupture.
A class 4 probe was a type of probe utilized by the Federation for conducting close-range scientific research.
In 2369, Benjamin Sisko wanted to use a class-4 probe to make a full analysis of a subspace rupture that appeared off of Deep Space 9. (DS9: "If Wishes Were Horses")
Class 4 probes were also used that year by the USS Enterprise-D to monitor the formation of a new star in the Detrian system. (TNG: "Ship in a Bottle") One was also launched the same year by the ship to investigate a molecular reversion field encountered by the shuttlecraft Feynman. (TNG: "Rascals") The USS Enterprise-D also lauched a series of class-4 probes to study the far side of a newly discovered Dyson Sphere, after the resue of Scotty from the USS Jenolan. (TNG: "Relics")
In 2371, the USS Voyager launched a class 4 microprobe to provide a distraction while healing the nucleogenic cloud being (VOY: "The Cloud")
Later that year, Voyager launched a class 4 probe in an attempt to distract a large, aggressive space-dwelling lifeform. The probe though only seemed to make it angrier. (VOY: "Elogium")
In 2372, a Jadzia Dax proposed using a class 4 probe to test whether an artificial wormhole, created by Lenara Kahn and her team, was stable enough to allow starship travel. Dax believed this class of probe would provide a more accurate assessment because its shielding was similar to that on a starship. (DS9: "Rejoined")
In 2373, a class 4 probe was used to transmit a location signal, as well as several farewell recording from the crew of the Defiant, after being trapped behind an energy barrier surrounding the planet Gaia. (DS9: "Children of Time")