On Essential Grid 4 the plugin and every update are free, but the premium add-ons, the premium templates and support need a yearly licence; premium add-ons and templates already installed on a site registered under Essential Grid 3 keep working, and new premium downloads need a yearly licence.
Import your purchase code for 50 % off the first year of a yearly licence, once per licence; the offer is open for six months from the 4.0 release. Your steps are on the upgrade page.