This paper proposes a methodology for testing the configurable logic of RAM-based FPGAs taking into account the configurability of such flexible devices. The methodology is illustrated using the XILINX 4000 family. On this example of FPGA, we obtain only 8 basic Test Configurations to fully test the whole matrix of CLBs. In the proposed Test Configurations, all the CLBs have exactly the same configuration forming a set of one-dimensional iterative arrays. The iterative arrays present a C-testability property in such a way that the number of Test Configurations 8 is fixed and independent of the FPGA size.