Jolla has in association with Rovio (company behind Angry Birds) released Angry Birds Stella Launcher. By installing this app you can turn your Nexus 4 or 5 User Interface look like Sailfish OS familiar from Jolla phone. This *does not mean* your phone has Sailfish OS inside. No no. Android is still there.
Please keep in mind that if you install this software you do it at your own risk. I'm going to try it and flash my phone again to get rid of it completely. Read the FAQ from Jolla first. You can also search for videos in YouTube, like this one (in French). And finally you can find Angry Birds Stella Launcher at Google Play.