Yes, we have done that, you can use one more WordPress installation as a single sign-on server.
https://softwaretestingboard.com (WordPress MultiSite)
All four site can connect to one common server, https://myaccount.publicityport.com to get the user authorized.
So, users do not really have to create a separate account for all sites.
For, WordPress you can create a single sign-on client. And for Q2A, you can use the plugin, https://github.com/PublicityPort/q2a-publicityport-login