The purpose of the IRS Letter 4464C is simple. It mainly informs the recipient two things.
First, the department has received the income tax return. Second, the person cannot expect to receive their tax refund anytime soon because it is still under review. In most cases, the delay is only 60 days. The IRS instructions for 1040-X say "File Form 1040-X only after you have filed your original return.". Generally, on this forum, the advice has been to wait until the original is fully processed before filing an amended return - the reason being if the IRS makes changes to the original then the amended is going to be wrong. Don't know the nature of the error you made. sometimes the IRS will catch it and adjusts automatically.
