I'm not sure where you found information regarding the 4th Mississippi becoming the 46th. Company K of the 46th Mississippi were the "Kemper Guards" from Kemper County. They were originally assigned to a Virginia regiment - 59th Virginia, until they were captured at The Battle of Roanoke Island in February 1862. They were eventually exchanged, and sent back to Mississippi where, in September, they were added to the 6th Battalion Mississippi Infantry, enabling that unit to reach regimental strength, subsequently becoming the 46th Regiment Mississippi Infantry. Unfortunately for research purposes, it will be difficult to tell where the John Kemp in this unit was from. He was a conscript (draftee), and as such could have been (and probably was) assigned to any unit from Mississippi that needed numbers. He was conscripted Aug. 21, 1862, and joined the unit on Oct. 9, 1862, from conscript camp. The 46th was at Vicksburg, and were surrendered with that garrison. They were given furloughs, and eventually exchanged Oct. 16, 1863. On Dec. 15, 1863, John Kemp was detailed as a company cook. The final muster roll on which his name appears is for Jan-Feb 1864. His status is shown as "absent without leave since Feb. 9, 1864." There are no further records. It is possible that he later returned, and that later muster rolls with his name are missing. It is simply impossible to tell from the records.