On Monday, Apple updated the iOS 8 Beta to Beta 4 and Mac OS X Yosemite to Developer Preview 4 (DP4). This article gives some useful links to learn more about iOS 8 and Yosemite.
For iOS 8 Beta installation steps, tips and more check out my blog article How To Safely Test iOS 8 Beta. For Yosemite Dev Preview installation steps, tips and more check out my blog article How To Safely Test OS X Yosemite Beta.
The upgrade of iOS 8 on my iPad Air took about 37 minutes, a little longer than Beta 3. The download took about 11 minutes and the rest of the time was on the device preparing, installing and rebooting. On an iPod Touch 5th gen (the latest model available) it took about 38 minutes, so nearly the same as on the latest iPad Air. The Yosemite update took about 19 minutes total.