Go 1.4 should be released officially next week. There’s currently a Go 1.4rc2 that’s very close. Windows builds will require Go 1.4 starting with v0.10.10. Some other developer-only features (currently regenerating the XDR files) may require Go 1.4 as well.
For other platforms Go 1.3 will likely continue to work for quite a while in the future, although the official builds will be made with 1.4.