Can I use Pencil+ 3 after purchasing or upgrading to Pencil+ 4?

You can use Pencil+ 3 with the plugin Pencil+ 3.11 (Ver4 Bundle) provided to users of Pencil+ 4.

Pencil+ 3.11 (Ver4 Bundle) is a plugin that runs using a Pencil+ 4 license. It can be used with 3ds Max 2015 / 2016 / 2017.