import sequtils, strutils

let t = parseInt readLine stdin

for _ in 0..<t:
  let
    xs = stdin.readLine.split.map parseInt
    l = xs[0]
    r = xs[1]
    a = xs[2]
    b = xs[3]

  if a > 0:
    echo a * r + b
  else:
    echo a * l + b