Bounds for Self-Dual Codes Over ℤ_4

New bounds are given for the minimal Hamming and Lee weights of self-dual codes over ℤ_4. For a self-dual code of length n, the Hamming weight is bounded above by 4[n/24]+f(n mod 24), for an explicitly given function f; the Lee weight is bounded above by 8[n/24]+g(n mod 24), for a different function g. These bounds appear to agree with the full linear programming bound for a wide range of lengths. The proof of these bounds relies on a reduction to a problem of binary codes, namely that of bounding the minimum dual distance of a doubly even binary code.