import Data.Array

po :: (Integral a) => a
po = 10^9 + 7

main :: IO ()
main = do
  [h, w, a, b] <- map read . words <$> getLine
  let
    fact = array (0,h+w) ((0,1) : [(i, i*fact!(i-1) `mod` po) | i <- [1..h+w]])
    combi n m = (fact ! n) `div` (fact ! m) `div` (fact ! (n-m))
  print $ sum [combi (h-a-1+i) i * combi (w-1-i+a-1) (a-1) | i <- [b..(w-1)]] `mod` po
