First of all, why do you think that there are four current-carrying conductors? In other words, why are you counting the neutral? Are there significant harmonics among the intended loads? If not, you only have three CCCs, and the derating factor of 80% does not apply.
But if you do have four CCCs, then you look up the ampacity of the #2 wire at its temperature rating (you didn't tell us what type of wire you are using). If for example you are using THHN, then at 90C the ampacity of a #2 is 130 amps. Multiply that by 80%, and you get 104. That is lower than the 75C rating of #2, so the ampacity limit is 104. Since your 70 amp load is below the ampacity of the conductor, you are good to go.