the defference between mazda fc rx7 ser. 4 and 5?

FC Series 5 is the facelift version of Series 4; the other way to say it is that Series 4 is the early version of FC, while the Series 5 is the later version of FC.
The major difference is the engine. In Series 4, the NA engine has 146 hp and the turbocharged version has 182 hp; for Series 5, the NA engine has 160 hp and the turbocharged version has an even 200 hp. Series 5 also have lighter rotary engine due to different alloy used for the engine block, plus a higher compression ratio.