"I am absolutely delighted to secure the signature of one of the most exciting young talents in the Premier League for a further four years," director of football Nick Hammond told the club's official website.
"The progress he's made in such a short space of time since joining us has been phenomenal, and the faith placed in him by the manager has allowed for that development.
"Shane now has a big opportunity and the challenge for him is to force his way into the starting line up on a regular basis in the coming season."
Meanwhile, Ingimarsson, Harper and Cox have put pen to paper on three-year contracts.
"As (manager) Steve Coppell has said, one of our biggest jobs this summer is to secure the futures of our existing players, and these deals are an important part of that process which will continue in the coming weeks," added Hammond.