In general intereration point of view, the 3C probe for an interaction between two specific regions that you identify a priori. In 4C, one fragment of interest is known, and you probe for all the potential fragments interacting with your fragment of interest.
We are using Hi-C in genome assembly and scaffolding. The Hi-C is an unbiased method that can identify all potential fragment interactions without any a priori knowledge of the interacting fragments.
In general intereration point of view, the 3C probe for an interaction between two specific regions that you identify a priori. In 4C, one fragment of interest is known, and you probe for all the potential fragments interacting with your fragment of interest.
We are using Hi-C in genome assembly and scaffolding. The Hi-C is an unbiased method that can identify all potential fragment interactions without any a priori knowledge of the interacting fragments.
I will recommend you to read this http://genesdev.cshlp.org/content/26/1/11.full.pdf+html for better understanding of the terms.