Karatsuba's recurrence for n-digit multiplication is

Karatsuba reduces the product of two n-digit numbers to THREE products of (n/2)-digit numbers plus linear additions. Hence T(n) = 3T(n/2) + Θ(n), which by master theorem gives O(n^log_2 3) ≈ O(n^1.58). Four sub-problems would be the naive split (giving O(n²) again) two would beat the actual O(n log n) for sorting, which Karatsuba does not do.