Fowl adenovirus (FAdV) serotype 4, recognized as the causative agent of hydropericardium syndrome (HPS) in chickens, causes substantial economic losses in poultry farming. To develop a simple, rapid, and reliable diagnostic method for the timely detection of FAdV-4 nucleic acid, we integrated the CRISPR/Cas12a system with recombinase-aided amplification (RAA). This approach enables visual detection of FAdV-4 with a sensitivity of one genome copy. The results can be obtained within 40 to 50 min without the need for complex instrumentation, making it ideal for remote field applications. Using this method, we investigated the prevalence of FAdV-4 in both common farm poultry and wild birds. Our results indicated that the FAdV-4-positive rate in wild birds was 51.19%, suggesting that wild birds may serve as specific reservoirs for this virus. In summary, we present a sensitive, swift, accurate, and inexpensive detection method for FAdV-4, along with an investigation of its epidemic situation in birds. Our study advances the detection and epidemiological understanding of FAdV-4 transmission among farm poultry and wild birds.