The idea is simple... Some time before the end of KH3, Xehanort sent away a replica to "explore the new world"... In the Dark Road scene, Young Xehanort and Eraqus are playing chess. Xehanort discusses 13 Lifetimes to Rebuild the World, and a 14th Lifetime to explore a new one. We know that Xehanort plus the true Organization XIII = 13 Xehanorts (including non-Xehanort characters with a Xehanort heart fragment). And later, there are 12 Replicas + Xehanort himself, also equals 13 Xehanorts. Xehanort is a man full of backup plans. Apprentice Xehanort already theorized about the other side; the recreation of the arc (Apprentice Xehanort sent Kairi in) was in Radiant Garden, maybe the machine itself was perfected? We know Xehanort worked with Vexen on the Replica program. This theory works because it preserves the finality of KH3 (Xehanort still passed away and accepted defeat before ascending to Kingdom Hearts above with Eraqus' heart). The biggest issue with normal post-KH3 Xehanort theory is that it undermines the ending of the entire Dark Seeker Sage. The 14th lifetime Xehanort allows Xehanort to be back without undoing his KH3 ending. He can still have memories/ experiences of Xehanort, but be based on Xehanort in his appearance (looks-wise), since he is a replica. He doesn't have to magically de-age either.

Xehanort in KH4 is Xehanort's 14th Lifetime, Not Post KH3 Xehanort or Dark Road Era Xehanort (Theory)