N1QL will be part of Couchbase Server 4.0 (CE and EE), which is currently in BETA (and so is N1QL, there is no “Stable/GA” version yet).
Note: It is recommended you preview N1QL with the 4.0 beta, but in case you’d want to preview it with a 3.x version it is maybe still possible to find
N1QL DP4(developer preview 4, a quite older revision) that runs as a separate process.