In this paper, an interferometric fiber optic vibration sensor based on a four-core optical fiber is described. When the light is coupled into the four cores, each core acts as a mutually coherent waveguide with the other ones, which allows obtaining an interference fringe pattern at the far field. Vibrating a section of the four-core optical fiber causes a path difference between the light beams guiding in the separate cores, which results in a shift in the fringe pattern. Such a mechanism allows one to relate the fringe shift to the vibration amplitude and frequency. In this study, a source, which is capable to generate 100 Hz frequency sound waves is attached to the optical fiber to maintain vibration of the section of the fiber. A single slit and a photodetector are used to detect the shifting of the fringe pattern that causes a change in the phase of the guiding light. When a He-Ne laser beam is coupled into the optical fiber, the structured fringe pattern is projected onto the slit behind the photodetector, then a small part of the fringe pattern is analysed. Thus, an interferometric fiber optic vibration sensor based on a four-core optical fiber, which has a simple structure and high sensitivity, is accomplished.