Based on the information you have provided and not interjecting any other outside factors that may apply:
Player 4's hand is dead and the 20,000 stays in the pot,
Player 5 can call the 20,000, raise or fold,
Players 1, 2 and 3 have the same options as player 5
Under no circumstances would I consider returning Player 4's raise, nor would I consider freezing the betting, nor would I consider the hand dead and return bets.
Expensive lesson for Player 4, but it is one not likely to be forgotten anytime soon.
Chet