Linking 4 WP installations

Hi

I basically want 4 installations and themes linked together.
Site1 being the main login site and users also need access to site2,3 and 4 without having to login to each site seperatly.
Do I need to share the database? or can i just edit the config code to use users from database X?
Also do I really need to add the line:
define(‘WP_ALLOW_MULTISITE’, true);
and if I do do I add this to all 4 sites?

I would love to have a little guidance from someone who has set up the themes to work alongside each other already.

A lot of information I have read refers to the multisite feature but I dont think this is what I want? As I am using 4 seperate installations on the same server.

I did get close by each install using the same database but with a different prefix, but I ran into trouble for the admin section and couldnt log in to the other sites admin dashboard.

A lot of info on here is old regarding this setup and Im not sure if works the same way with new versions of WP.
Hope someone can help.
Mark