Well, depends on the kind of c4d. There's four main types.
There's effect c4ds, that look like this, usually:
[spoiler:3n6x5v5g]
[/spoiler:3n6x5v5g]
Effect c4ds are used for extra effects in a sig, but can also be used as background elements. These are best if set to a lightening blend mode, like lighten or screen.
A sig using effect c4ds:
***
Solid c4ds look something like this:
[spoiler:3n6x5v5g]
[/spoiler:3n6x5v5g]
Solid c4ds (that's not really the official name for them) are used mainly to fill up backgrounds, and can create a snotload of depth if used correctly. Best kept on normal blend mode.
A sig using solid c4ds:
***
Fractal c4ds often are similar to this:
[spoiler:3n6x5v5g]
[/spoiler:3n6x5v5g]
I don't use fractals very often but they, to my knowledge, are best used for giving a signature some texture, along with a subtle effect. Either that, or can be used for making a background. The blend mode of choice changes between different fractals.
A sig using fractal c4ds:
And wireframe c4ds, probably the rarest, resemble this:
[spoiler:3n6x5v5g]
[/spoiler:3n6x5v5g]
These are used as background implements, they can also spice up a dull or solid area of a tag. You really only use these on a darkening opacity, usually "multiply."
A sig using wireframe c4ds:
***
Hope this helps!
***These are tags made by me, and everything else in this post (including the c4ds) is not made by me. I do not claim ownership on any of the non-asterisked pieces.