ISO 9974-4:2016 specifies dimensions and performance requirements for metric external hex and internal hex port plugs for use with ISO 9974-1 ports.
Port plugs in accordance with this document can be used at working pressures up to 63 MPa [630 bar[1]]. The permissible working pressure depends upon the plug end size, materials, design, working conditions, application, etc.
Conformance to the dimensional information in this document does not guarantee rated performance.
NOTE The Introduction of this document gives recommendations for ports and stud ends to be used for new designs in hydraulic fluid power applications.

[1] 1 bar = 0,1 MPa = 105 Pa, 1 MPa = 1 N/mm2.