GeekSn0w is a popular iOS 7.1.2 Jailbreak utility that's built upon winocm's opensn0w project and Geohot's limera1n exploit. GeekSn0w can Jailbreak all iPhone 4 models on Apple's latest public firmware, iOS 7.1.2. For a detailed explanation as to why Geeksnow only supports the iPhone 4, visit Jailbreak Evasion Info's constantly updated Jailbreak iOS 7.1.2 category, which also outlines the distinctions between Tethered and Semi Untethered Jailbreaks. To Jailbreak with GeekSn0w, simply download the iOS 7.1.2 tool to get started.

Link To Download for:

Official tutorial (thanks to iCrackuriDevice);
Also if you want to win awesome rewards by downloading Free Apps on the App Store, make sure to check out FAL

Written guide

GeekSn0w supports: - iPhone 4