Abstract: The 4-velocity as a crucial node is linked to two aspects in the special relativity. One aspect is the relative 4-space-time due to the postulate of the constancy of the light speed, while the other aspect is the covariant mechanics and electrodynamics laws due to the postulate of the relativity. Teaching practice for a long time tells us that an emphasis and enhancement in the teaching in the 4-velocity and its connection role can inspire learners to understand the ins and outs of the whole theoretical system, and facilitate them to lay a solid foundation of physics.

Key words: electrodynamics, special relativity, 4-velocity, 4- moment, 4-current density, 4-electromagnetic potential