  R. Hofer and A. Winterhof proved that the 2-adic complexity of the two-prime
(binary) generator of period $pq$ with two odd primes $p\neq q$ is close to its
period and it can attain the maximum in many cases.
  When the two-prime generator is applied to producing quaternary sequences, we
need to determine the 4-adic complexity. We present the formulae of possible
values of the 4-adic complexity, which is larger than $pq-\log_4(pq^2)-1$ if
$p<q$. So it is good enough to resist the attack of the rational approximation
algorithm.
