Small 5 Fr guiding catheters are now used in percutaneous coronary intervention (PCI). However, when treating severely calcified and/or tortuous complex coronary lesions, the back-up support of a 5 Fr guiding catheter is often insufficient. We previously developed a 4 Fr child catheter (Kiwami ST-01; Terumo Corporation) and proposed a 4 Fr double-coaxial technique ("mother-child" configuration) to facilitate stent implantation in treating complex coronary lesions. In this manuscript, we report 2 cases of 5 Fr transradial coronary intervention in which stent delivery failed with conventional techniques. In these 2 cases, our 4 Fr double-coaxial technique was employed, and stents were successfully delivered to the target lesions. To the best of our knowledge, this is the first report demonstrating the potential of the 4-in-5 technique during 5 Fr PCI.