Not sure what other skins will be available in skin pack 4 but the borderlands characters were confirmed on borderlands Facebook page (Second picture in comments) : r/Minecraft360
Borderlands Minecraft Skins. Download for free at SuperMinecraftSkins
Borderlands 2 - CL4P-TP Minecraft Skin
Borderlands and Minecraft Meet Again! - Gearbox Software
Borderlands 2 - Zer0 (with or without the 0) Minecraft Skin
borderlands | Minecraft Skins
Borderlands Minecraft Skins | Page 4 | Planet Minecraft Community
Borderlands Minecraft Skins | Planet Minecraft Community
Guide: How to Download and Upload Minecraft Skins - BagoGames
Borderlands 3 - The Beastermaster // Flak Minecraft Skin