The Go AppEngine runtime environment uses Go version 1.2 as you can read it on the Go Runtime Environment page.
A beta version of 1.4 runtime version is available, you can read more about it here:
go1.4beta is now ready for testing
If you want to use the 1.4 beta runtime, you have to specify it in your app.yaml config file like this:
api_version: go1.4beta
Note that even though Go 1.4 is out (current version is 1.4.2), the AppEngine runtime version of 1.4 is still not released.
Quoting the relevant part:
Go 1.4 has not yet been released, so the go1.4beta runtime is built
from the release-branch.go1.4 of the core Go tree, and may be updated
as Go 1.4 approaches.
We will not be providing a beta version of the Go 1.4 SDK. You should
use goapp deploy (or appcfg.py) from the normal Go App Engine SDK to
deploy your app.
To use go1.4beta, update your app.yaml file to use the new api_version:
api_version: go1.4beta
After Go 1.4 is released it will become the basis for api_version
'go1', as normal, and 'go1.4beta' will be retired soon afterward. We
expect this to happen early next year.
