5310：Souvenir

Today is the 1st anniversary of BestCoder. Soda, the contest manager, wants to buy a souvenir for each contestant. You can buy the souvenir one by one or set by set in the shop. The price for a souvenir is $p$ yuan and the price for a set of souvenirs if $q$ yuan. There's $m$ souvenirs in one set.

There's $n$ contestants in the contest today. Soda wants to know the minimum cost needed to buy a souvenir for each contestant.

There are multiple test cases. The first line of input contains an integer $T$ $(1 \le T \le 10^5)$, indicating the number of test cases. For each test case:

There's a line containing 4 integers $n, m, p, q$ $(1 \le n, m, p, q \le 10^4)$.

For each test case, output the minimum cost needed.