To OP, they developing AoE4, not 5
Relic is likely on contract for more than one AoE game.
Based on Relic job announcements working on AoE's franchise not only AOE4:
https://forums.ageofempires.com/t/relic-to-hire-narrative-lead-for-aoe-franchise/37166
Job archive:
http://archive.ph/DPJuY
Relic posted this job (AoE - FRANCHISE NARRATIVE LEAD) over one year ago:
Highlight, Quote:
Age of Empires – Franchise Narrative Lead
For over 20 years, Age of Empires has delighted fans of all ages with its celebratory approach to history and ageless game mechanics that have delivered a large, diverse, and world-wide gaming audience of millions of dedicated fans. Age of Empires IV marks a return to this highly anticipated and critically acclaimed Franchise.
Your role will be to shepherd the strategic view of the narrative of the Age of Empires franchise at Relic Entertainment, working with our fantastic partners at Microsoft to define the narrative goals and focus of the franchise. In addition, you will work to ensure narrative consistency in the game, and explore and drive the delivery of narrative within the Age of Empires brand at large.
You and your team will deliver an amazing experience for our players and community by mining our shared world history for great stories to present in unique and fulfilling ways.