Code 4 indicates that the distribution came from an inherited IRA maintained for the benefit of your Mom, not from your Mom's SEP-IRA. Because IRA custodians typically do no make any distributions until the funds have been moved, the code-4 Form 1099-R implies that your Dad's IRA was first transferred to an inherited IRA for the benefit of your Mom, that a distribution of Dad's RMD was made from that IRA, then that IRA was transferred to your Mom's SEP-IRA. That intermediate inherited IRA account was an unnecessary step since your Dad's RMD could have just been taken from your Mom's SEP-IRA after a transfer of your Dad's SEP-IRA directly to your Mom's SEP-IRA.
Check the account numbers shown on the Forms 1099-R.
