Does Translink iLink Zone 4 cover travel across all of Northern Ireland?

Yes. Translink describes Zone 4 as covering all its bus and rail services throughout Northern Ireland.

That is broader than the numbered local zones or the separate North West zone. It is a Northern Ireland product, not a blanket ticket for international or cross-border services.

Practical advice: Check the ticket rules before assuming a Zone 4 pass will cover a trip beyond Northern Ireland.