import sys

input = lambda: sys.stdin.readline().rstrip("\r\n")
rint = lambda: int(input())
rlist = lambda: list(map(int, input().split()))


def solve():
    N, D = rlist()
    A, B = rlist()
    ans = A * min(D, N)
    ans += B * (N - min(D, N))
    return ans


for tc in range(rint()):
    solve()