Pricing compares the plans. Your theme’s users get Essential Grid 4 and every update for free; for the premium add-ons, the premium templates and support, they need a licence of their own.
An Extended licence from Envato counts as a CodeCanyon licence: it keeps working for Essential Grid 3, add-ons included; on Essential Grid 4 support and new premium downloads need a yearly licence; what is already installed keeps working. Import its purchase code into your account before you choose your plan: you get 50 % off the first year, once per licence; the offer is open for six months from the 4.0 release.