I've been using Invision 4 for years along with a 3rd-party Invision plugin called WordPress SSO by Stuart Silvester. This plugin allows people who login to my Wordpress membership to then be automatically logged into to my Invision community, and it handles mapping WP user roles over to the matching Invision groups.

But I'm not ready to upgrade to Invision 5, and I'm not sure what my options are for implementing this same WP to Invision auto sign on?

You would need to speak to the author of that plugin to ask the question. You can set up basic login with wordpress using the software, but it wont be automatically logged in