DNA topoisomerase IV (the parC-parE complex) converts the concatenated DNA into decatenated DNA. It also relaxes supercoiled DNA. The DNA Topoisomerase IV Decatenation Assay is based the principle that the decatenated DNA is separated from the concatenated DNA by a spin-column or filtration process.  The concatenated DNA stays on the column or filter, while the decatenated DNA is eluted.  The eluted DNA is quantified by fluorescence. A high throughput assay based on the DNA relaxation activity is also available.

DNA decatenation assays (96-well plate filter format): 

DNA decatenation assays for topoisomerases in 96-well plate assay format are available for high-throughput screen of topoisomerase inhibitors.

High throughput DNA topoisomerase IV assays (96-well plate solution format) 

References:

Narayanan S et al,A cell cycle-controlled redox switch regulates the topoisomerase IV activity. Genes Dev. 29(11):1175-87 (2015).