Chrome OS: Currently, if you want to add a lock screen to your Chromebook, you either have to use your Google password (which should be long) or pair it with a smartphone. Soon, you may be able to use a PIN instead. As news site OMG Chrome has found, the latest Canary build of Chrome OS -- which features only the most bleeding edge updates -- now gives users the option of unlocking with a PIN instead of a password. This only applies to the lock screen, of course. The first time you boot up your computer, you'll need to enter your account password, but you won't have to do it again every time you close the lid.
If you're not feeling adventurous enough to try out the Canary builds, which are highly experimental and subject to change, you can wait for this feature to rollout to your device in the near future. While features in Canary builds are subject to change, this is such a needed addition that it seems unlikely to be cancelled entirely.
PIN Unlock Is Coming to Chrome OS [OMG Chrome]