In this paper, we present a 128~2048-point variable-length FFT processor for 4 × 4 MIMO-OFDM systems. In general, we must use n FFT processors for the n × n MIMO-OFDM system, and only 1/r operation time is employed for radix-r pipeline butterfly processor. This increases hardware cost and reduces hardware efficiency in VLSI implementation. Therefore, we propose radix-42 algorithm to deal with four data sequences simultaneously and a butterfly sharing technique to improve the hardware utilization. The design and implementation results show that the proposed FFT processor can achieve 83.3MHz frequency with 3.19mm2 core area using a standard 90nm technology.