The size 4 and 5 designate the body size of the electrode. A #4 electrode has a ½” body diameter with a 3/16” face diameter. A #5 electrode has a 5/8” body diameter with a ¼” face diameter. These are sizes established by the Resistance Welding Manufacturers Alliance (RWMA) and are published in Bulletin 16 Section 18. All manufacturers produce to these standards here in the USA. There also is a size 6 and 7. Size 7 is rarely used in cap form. The charts below show data for RWMA caps and full size electrodes.
Female Cap
Male Cap Electrode
Full Size Electrode
Metric standards are also available for Metric caps.
Ref: RWMA Manual 4th Edition
ISO Std 5821