According to the Star Trek: The Next Generation Technical Manual (p. 116), the class-4 probe is a stellar encounter probe. It is a modified class-3 probe with extended fuel supply allowing a 3,500,000 km range and propulsion supplemented with a driver coil. Sensors include triple redundant stellar field and particle detectors and a stellar atmosphere analysis suite. The probe is also equipped with six ejectable subprobes. See also: class-4 microprobe.
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 USS Defiant, after being trapped behind an energy barrier surrounding the planet Gaia. (DS9: "Children of Time")