Source? They said they transitioned to full production after Blood & Wine, but they've been in development since Witcher 3 base game launched iirc
http://www.neogaf.com/forum/showpost.php?p=234161655&postcount=1
Should be treated just as a rumour though, but some other places have reported the same. Probably based on the same rumour =)
Edit: Yeah those few articles seem to state that post as a source.