For the first time in April 2015, KitKat OS grabs the most popular Android OS title from Jelly Bean. The data is released officially by Google, for the worldwide market.
According to Google, 41,4 percent of Android users installed KitKat on their devices. While the combination of all Jelly Bean OS (4.1, 4.2, 4.3) is now at 40,7 percent. For details, you can see the following chart:
So, if you're still using Jelly Bean, no worries. You still have so many friends out there. Of course, if you want to buy a new Android device, better go for the latest OS.