Four characters are the last four of your serial number from
https://supportprofile.apple.com
The MDxxxXX/A number is your part number from the Your Order is being Processed email Apple sent you on Friday morning.
Please cut the info below and paste it into a new post when you update, adding the last four or part number for missing parts. There will be more than one part for Europe/World/Canadian phones, so just append to the list for that model:
64GB Black (at&t or Canadian carriers) - F38W
MD644LL/A
64GB White (at&t or Canadian carriers) - F38Y
MD645LL/A
32GB Black (at&t or Canadian carriers) - DTTQ
MD636LL/A
32GB White (at&t or Canadian carriers) - DTTR
16GB Black (at&t or Canadian carriers) - DTTN
MD634LL/A
16GB White (at&t or Canadian carriers) - DTTP
64GB Black (Sprint) - ????
64GB White (Sprint) - ????
32GB Black (Sprint) - F8GK
32GB White (Sprint) - ????
16GB Black (Sprint) - F8GH
MD656LL/A
16GB White (Sprint) - F8G7
64GB Black (Verizon) - ????
MD664LL/A
64GB White (Verizon) - ????
32GB Black (Verizon) - ????
MD658LL/A
32GB White (Verizon) - F8H6
16GB Black (Verizon) - F8H2
MD654LL/A
16GB White (Verizon) - F8H4
64GB Black (European/World Model) - F39C
64GB White (European/World Model) - F39D
32GB Black (European/World Model) - DTWG
32GB White (European/World Model) - DTWH
16GB Black (European/World Model) - DTWD
16GB White (European/World Model) - DTWF