Upgrading is simple, but does require a few tweaks.
The good news is you can upgrade and play with v4 for a particular project without having to change all of your gulp files.
This version features some nice additions to the build tool but does also feature some substantial breaking changes.
You will need to remove your current gulp global package before installing v4 in order to do an upgrade.
This command removes your current global package and installs v4 from the gulp-cli 4.0 branch.
This is ideal if you want to see what gulp v4 means for your and your workflow before taking the big plunge.
Note: You need to upgrade the global package to use v4, but v3 will still work for each local project where needed.