Hmmmm. Good one. Maybe request an amendment to take into account the liquidation of the IRA (I think Eric has talked about compromising a pending offer). Otherwise you might have to let the client know that he/she needs to beg from family and friends to get the money to pay the estimates.
Does the client owe that money? If the P&L shows they wont owe that than I would push back. If they do and failed to maintain compliance you can try but the IRS will not consider the IRA spent and agree to amend the Offer because I bet the offer included a current tax amount they were supposed to pay. You can try but do not be surprised if they refuse to agree to an amended offer.
I would have them get the IRA, pay in the additional amount, and hopefully make up the $4K balance before the final offer payment needs to be made if accepted.