687 B | 677 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
BLANK = " "
#inp = input
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
T_a, T_b, V_a, V_b = mii()
X = T_b * V_b
ans = 1111111111111111111111111
for i in range(V_a + 1):
j = V_a - i
# i: 숙련된
# j: 초보
ans = min(ans, max(X + T_a * i, j * T_a))
p(ans)
if __name__ == "__main__":
tc = ii()
for t in range(1, tc+1):
ret = solve()