Apple has just released the iOS 4 Gold Master Candidate. The first developers preview version of iOS 4 GM, along with SDK for iPhone, iPod touch and iPad was made available for download. If you have a membership of the iPhone Developer Standard or Enterprise Program, you can download iOS 4 GM firmware for iPhone 3G, 3GS & iPod Touch 2G & 3G now! Recall that GM (Gold Master) is a version that can be considered equal to the final.
A whopping list of 1500 new features is riding on iOS 4 Gold Master Candidate including multitasking, a unified inbox, app folders, data detectors, Bing search integration. The first-gen iPod is left out entirely, and the 3G won’t do multitasking unless you forcibly enable multitasking on your iPhone 3G after jailbreaking iOS 4 and editing N82AP.plist.
If you want to install iOS 4 GM firmware on your iPhone then use the following link to download iPhone OS 4 firmware.
Download iOS 4 Gold Master firmware
Note:You NEED to register your device ID with a Dev Account to use it.