When I first heard that Chrome OS will be able to run Android apps, I assumed that Chrome OS will include Android's runtime and it will be able to launch any Android app. That's not the case, Android apps are manually ported to Chrome, so that's why Google's first set of apps only includes Duolingo, Evernote, Sight Words and Vine.
Bringing more powerful apps to Chrome OS is a great idea. Making it easier to port existing mobile apps to Chrome encourages developers to go beyond web apps and write native apps that work offline, include hardware integration and work outside of the browser. While cross-platform web apps are still useful, the new Chrome apps can bring some missing features that people expect to find in native apps. "These combine the best of websites and native applications — they're available offline, are always up to date, and they can communicate with devices like USB drives & Bluetooth speakers," explains Google.
"These first apps are the result of a project called the App Runtime for Chrome (Beta), which we announced earlier this summer at Google I/O. Over the coming months, we'll be working with a select group of Android developers to add more of your favorite apps so you'll have a more seamless experience across your Android phone and Chromebook," informs Google. You can tell Google what Android apps you'd like to be ported to Chrome.
For now, the first 4 apps can only be installed in Chrome OS, but I'm sure that Google will add support for Chrome in the near future.