When playing music in an ensemble, musicians need to precisely coordinate their actions with one another. As shown in our previous studies on guitar duets, interbrain synchronization plays an essential role during such interactions. In this study, we simultaneously recorded electroencephalograms from four guitarists during quartet playing, to explore the extent and the functional significance of synchronized cortical activity across four brains. We found that hyperbrain networks based on intra‐ and interbrain connectivity across four brains dwell on higher frequencies for intrabrain communication and on lower frequencies for interbrain connections. The hyperbrain networks show small‐world topology, with a tendency to become more random at lower frequencies and more regular at higher frequencies, such that local efficiency increases and global efficiency decreases with higher frequencies. We identified two different types of information flow within the hyperbrain networks—intra‐ versus intermodular—which are based on hyperbrain modules that include nodes from two, three, or even four brains. Furthermore, we found that hyperbrain networks are unstable and change their structure over time, often as a function of musical context. Our findings demonstrate complex hyperbrain network interactions in a guitar quartet and point to mechanisms that support temporally coordinated joint action.