If you're a bass player and have got scores for Bb instrument. Read them in C clef fourth line (I don't know how you call it in English). …
I think you mean G clef on the fourth line from the top (standard violin clef like the top staff of a piano score).
C clef is mostly used as alto clef for viola (da braccio) or tenor clef for violoncello, bassoon and trombone (classical) to avoid too many ledger lines.
The clefs are named after the notes that get marked. For the G clef it is the spiral, forming a g with the lower part of the clef, the upper part of this clef is only decoration, for the C clef the vertical center of the clef, a residue of a c, the wings above and below are only decorative, and for the bass or F clef the line between the two dots which are the residue of two horizontal lines forming an F with the bowed vertical line.
If you really meant a C clef on the fourth line from the top, that would be the mezzosoprano clef (the range without needing ledger lines is that of a mezzosoprano (well, closer to an alto these days).
In 16th Century Music (1501-1600) where early music typesetting started with elements that were single music notes on a staff each note duration and position needed a separate element, ledger notes would need even more elements, so to avoid them it was simpler to change the clef to keep the notes in the range of available typesetting elements. So often C or F clefs move up or down (mostly down) or a G clef moves or changes to a C clef and back. Often in the "middle" of a phrase.
Just an explanation where the clefs come from and why they were needed/used.
Nowadays we hardly need to be able to read more than three clefs for a certain instrument (bass, tenor or alto and violin), in many cases only one or two.