“The main place should be given, all things being equal, to Gregorian chant, as being proper to the Roman Liturgy.”

Here is the new translation of the General Instruction of the Roman Missal, 2011. This will appear in the Third Edition of the Roman Missal. This is the document that renders option 4 as “another liturgical chant that is suited to the sacred action, the day, or the time of year, similarly approved by the Conference of Bishops or the Diocesan Bishop.” For more on the implications, see this post.