write a go solution for Description:
The famous store "Second Food" sells groceries only two days a month. And the prices in each of days differ. You wanted to buy n kilos of potatoes for a month. You know that on the first day of the month 1 kilo of potatoes costs a coins, and on the second day b coins. In "Second Food" you can buy any integer kilograms of potatoes.

Fortunately, "Second Food" has announced a promotion for potatoes, which is valid only on the first day of the month — for each m kilos of potatoes you buy, you get 1 kilo as a gift! In other words, you can get m+1 kilograms by paying for m kilograms.

Find the minimum number of coins that you have to spend to buy at least n kilos of potatoes.

Input Format:
Each test contains multiple test cases. The first line contains the number of test cases t (1<=t<=10,000). Description of the test cases follows.

The first line of each test case contains two integers a and b (1<=a,b<=10^9) — the prices of 1 kilo of potatoes on the first and second days, respectively.

The second line contains two integers n and m (1<=n,m<=10^9) — the required amount of potatoes to buy and the amount of potatoes to use the promotion.

Output Format:
For each test case print one integer — the minimum number of coins that you have to pay to buy at least n kilos of potatoes.

Note:
In the first test case, on the first day you buy 1 kilo and get 1 more for a promotion. On the second day, you can buy 1 kilo of potatoes. Thus, you will spend 5+4=9 coins in total.

In the second test case, on the first day you buy 2 kilo and get another 1 more for a promotion. This way you will spend 2*5=10 coins.. Output only the code with no comments, explanation, or additional text.