FindInstance[a^3+b^3+c^3-d^3 == 0, {a, b, c, d}, Integers].

FindInstance[a^3+b^3+c^3-d^3 == 0, {a, b, c, d}, Integers,2].

Select[Na, i = RandomInteger[{0, 1000000}]; j = RandomInteger[{0, 10000}];
k = RandomInteger[{0, 10000}]; l = RandomInteger[{0, 10000}];
Na[[i]] = CubeRoot[Na[[j]]^3 + Na[[k]]^3 + Na[[l]]^3]].