Hi Patrick,
Please refer to this post to udnerstand screen and control id overwrite entries
4001 (Payment cards tab - also note the program name is SAPLV60F here), 5002 (conditions tab) and 4002 (all other tabs) are part of Doc header details screen. Hence they are mapped to X00X.
5003 (conditions tab) and 4003 (all other tabs) are part of item data screen. Hence they are mapped tp Y00Y.
Yes, you can map to anything like XXXX, YYYY, ZZZZ.
New program name is required when your mapped entries differ in old program name. In this case 4001 has SAPLV60F whereas 5002 and 4002 has SAPMV45A. So it is mapped to SAPXXXXX.
Always note that Personas flavors are identified by combination of tcode+program name+screen number. So even if you have used same codes or program names in other transaction, they dont clash.
Hope that makes sense.
Thanks
Chinthan