A refractive index sensor based on tapered four-core fiber is proposed in this paper. With the aid of a hydrogen-oxygen flame, the four-core fiber was progressively tapered between two segments of a multimode fiber. By simulating the light field of the sensing part, we prove that the tapered four-core fiber will produce stronger interference effect. Experimental results show that the sensor has a maximum refractive index sensitivity of 298.307 nm/RIU and a linearity of 99.806%. The sensor is also insensitive to temperature, which effectively avoids the cross-sensitivity effect between temperature and refractive index. With its small size and high sensitivity, the sensor is expected to have potential applications in biological and chemical sensing.