Line 4a has been the source of much controversy. Line 4a is supposedly the total of all distributions from IRAs, 401(k)s , pensions and annuities, etc. The problem is that some types of distributions are supposed to be reported here and others are not. Try reading the instructions for Line 4a to see what a confused situation it is.
But note that line 4a is not the taxable amount, line 4b is. So, in a sense, it does not matter what is being reported on 4a. Given the confused state of the instructions, I have trouble believing that the IRS will pay any attention.
And note that every distribution that was reported on a 1099-R to you was also reported to the IRS, so they have the information needed to construct what the number should be anyway.
So, in short, don't worry about justifying what the value for line 4a is.
