We’ve already seen several sub-second boot times for embedded Linux systems with platforms such as Beagleboard (TI OMAP 3530) and an AllWinner A10 based device, but I had never seen fast boot optimization for Android before, or I just forgot… Typically Android boots within 20 to 40 seconds on most devices, but AllGo Embedded Systems has optimized Android Jelly Bean boot time for their IVI (In-Vehicle Infotainment) platform running a media player called RACE.
Charbax interviewed then at CES 2014, where they demoed the 4 seconds (actually 4.2 seconds) boot on Texas Instruments OMAP5 from reset and power, as you can see in the video below. (The video starts at 2:20 to show the demo directly).
Beside Texas Instrument OMAP5, the solution is also available for Freescale i.MX6. 0xlab did some boot time optimizations for Android dropping the boot time from 30 seconds to 15 seconds. AllGo Embedded Systems did further optimizations, and since the system is embedded they probably did remove some unused components to further boost the boot time.
The company has an Android fast boot page, but with a few more details.