A class-4 probe was a type of probe utilized by the Federation for conducting close-range scientific research.
Uses
In 2369, a series of class-4 probes was also launched by the USS Enterprise-D to study the far side of a newly-discovered Dyson sphere, after the rescue of Montgomery Scott from the USS Jenolan. (TNG: "Relics") Later that year, the starship also launched a class-4 probe to investigate a molecular reversion field encountered by the shuttlecraft Feynman. (TNG: "Rascals")
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")
In 2371, USS 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")