Node 4 Support?

Does it make sense to support Node 4 until it reaches end of life? When installing via yarn, I'm getting:

```bash
error safe-env@1.2.0: The engine "node" is incompatible with this module. Expected version ">=6".
error Found incompatible module
```

Start by locating the package engine declaration that produces the reported Yarn error and inspect the existing environment-variable handling entry point. Try installation and the available tests under Node 4; done means the package installs with Node 4 and its existing behavior remains covered.

Written by the indexing model from the issue text.