Can no.4 fishing line be used as a sub-line?

No.4 fishing line cannot be used as a sub-line, because it is too thick and not soft enough, and the hook cannot be tied tightly.

different wire numbers represent different wire thicknesses, and the larger the number, the thicker the wire and the greater the tension (the same paragraph).

The main line refers to the fishing line from the pole to the splay ring, and the sub-line refers to the fishing line from the splay ring to the hook. Generally, it is required that the main line has higher density, better water cutting, softer sub-line and better recovery.