LNF-ISC4_8A is an ultra-low insertion loss cryogenic isolator operating in the 4-8 GHz frequency range. The isolator has been designed from ground up to meet the strict requirements of ultra-low temperature physics research. The gold plated OFHC copper body ensures minimum loss and that this loss reaches the lowest possible temperature to minimize thermal noise. The isolator is packed in a slim coaxial module using industry standard SMA connectors. The module measures 22.3*24.6*10.2 mm excluding the connectors. Each unit is delivered with actual test data taken at 77 K. Insertion loss improves with 0.07 dB when cooled from 77 to 3 K; all other parameters are unaffected.