ISO 9001:2015 sets a minimum standard whereas ISO 9004 shows you how much progress you can make towards best practice.It provides a much broader organisation view of best practice.

ISO 9001:15 now includes customers and stakeholders and ISO 14001:2015 are written in a consistent fashion so that it is easier to use them together in developing an integrated management system (IMS:2015)

Example: ISO 9001 is concerned that employees has the required skills and competencies to perform tasks which affect quality whereas ISO 9004 is concerned with all aspects of selecting, recruiting, inducting, training and retaining good employees.