Do you think you can really hide it from him? If he knows you have one they can issue a supenea to your employeer and get the information this way. He is entitled to 1/2 of the 401 k money that was put into the account during your marriage plus one half of the increase or in some cases 1/2 of the decrease.
If you hide it and he finds out after the divorce proceedings the entire divorce can be legally reopened and then a new settlement honored by a judge. I wouldn't do it, I would spend the money first befor I would consider hiding it. Just think you could get a divorce and find out that the settlement you got was null and void....I think this would be terrible. Freedom is worth so much more than a 401 k, no matter how much is in it.

they are wanting to go back to seperation date and I took that money and purposely hid it recently , will i be responsible for what was in there at the seperation date.

yes. Was the money that was in there on the date of separation collected entirely during your marriage? If yes, he is entitled to half, if no, it will only be half of the portion that was added after marriage plus the gain or loss.
If you took it out after separation they will see this transation if they supenea the records. They may just ask for the balance at separtation, if so they will get the information prior to you hiding the money. Either way they will find out, just tell them the truth it will look better to a judge if it has to go to court. Hiding assets is a very serious issue in a divorce, yes people do it and get away with it and other try and are found out, why risk it? Like I said previously, no amount of money is worth my freedom.