You shouldn't mismatch tires. Different compounds, pattern, and handling characteristics. While you could probably get away with it on the street, if you are heading for the track then don't cheap out and replace all four with matched pairs.
I don't know about the S-04s (reviews on TireRack seem decent), but the different threadwear rating tells me that they will heat up at a different rate then the RE-050s. You don't want to be on the track with your front & back pressures suddenly getting all wonky.
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