From Halopedia, the Halo wiki
Strilun IV, like other worlds such as Kamchatka and Zehar VI, was inhabited by the Forerunners. During their reign, the Lifeworkers held interest in Strilun IV due to its status as an ice world - building vast rehabilitation complexes within the world's glacial layers.
During their expansion, the Covenant found Strilun IV and settled the world, building multiple bases on its surface surrounded by Shuku-pattern sentry turrets and A'uo'i-pattern sensor emplacements.
Strilun IV is the setting for the Halo 3 multiplayer map Snowbound. The map's location was revealed thanks to the Halo Encyclopedia (2022 edition). On page 359, Strilun IV is mentioned as an ice world housing Forerunner complexes similar to Kamchatka and Zehar VI (the setting of the map Waterfall).
In the June 2022 edition of Canon Fodder, 343 Industries confirmed that the setting of Snowbound was hidden within the book's pages, though left the specifics of this lore ambiguous as to encourage fans to uncover the mystery for themselves. The connection is confirmed on page 301, with the caption for the Shuku-pattern turret confirming the world as Strilun.