from math import ceil, floor

n = int(input())
for lol in range(n):
    numss = input().strip().split()
    nums = [0] * 4
    for i in range(len(numss)):
        nums[i] = int(numss[i])
    if nums[0] <= nums[1]:
        print(min(nums[0]*nums[2], nums[1]*nums[3]))
    elif nums[2] <= nums[3]:
        print(nums[0]*nums[2])
    else:
        print((abs(nums[0] - nums[1]) + nums[0]) * nums[3])

        '''flor = (nums[1]*nums[3] + nums[0]*nums[2]) / (nums[3] + nums[2])
        one = max((ceil(flor) - nums[1]) * nums[3], (nums[0] - ceil(flor))*nums[2])
        two = max((floor(flor) - nums[1])*nums[3], (nums[0] - floor(flor))*nums[2])
        result1 = one + ceil(flor) * nums[3]
        result2 = two + floor(flor) * nums[3]
        print(min(result1, result2))'''