X-4 is the reinforcement list. Not a member of the CB Highl, byt tabbed to go to them if they required reinforcements.
13 Bn is just a designation within the reinforcement stream, there typically appeared to be 11, 12 and 13 Bns within a Cdn Base Reinf Bn or Base Reinf Group (this is probably the CBRB of your previous post).
Detailed organization of the reinforcement units is not well understood as far as I can tell.
You beat me to it. For example, Appendix B to Report No. 22 by the Historical Officer, Canadian Military Headquarters "Situation of the Canadian Military Forces Overseas, Spring 1944" shows 719 male officers, 68 nurses and 10826 male NCMs in the X-4 Reinforcement category in the Central Mediterranean Forces.