Webpacker 4.x support

[shaundr]

Webpacker's master branch is at 4.0.0.pre.3

The readme here states that there is support for Webpacker 3.x, 2.x, 1.1+

Are there plans to support 4.x? I do not see any open issues or branches.

[ttanimichi]

Can you take a look at #918?