You are in essence looking to create a break-out box for your 4-pair cable or patch cord. That is the term of art that you are searching for. Breakout boxes are used in many applications to allow you to get access to, and test, individual leads in a cable.
For your first application, use two 8P8C jacks, installed in a 2-port faceplate, screwed to a single-gang box. Plug your patch cords into the two jacks, so that the two jacks are in series with your devices. Be sure to use either 568A or 568B wiring protocol for both jacks. Inside the box, you can run 8 individual leads from jack #1 to jack #2. You can manipulate the leads any way you want, for your needs. You can open, short or cross them, as necessary.
For your second application, you need a Western Electric (made by Amphenol) or equivalent KS-19252 List 1, 2, 3 or 4, "3-way bridging adapter" that will allow three 25-pair cables to merge in one adapter. These adapters are small, about 3" by 3" by 5", and are comprised of three 25-pair Amphenols in parallel. See BSP section 461-200-102 as a reference.
List 1 is CPC, list 2 is PCP, list 3 is PPP and list 4 is CCC, where P stands for plug (male) and C stands for Connector (female).
(As a mnemonic, we also use slang for parts of the human anatomy to refer to the C and P, but since this is a G-rated forum, "the solution is left to the intuitive reader.")
The specific list# that you need depends upon the gender of the mating cables that you are dealing with. You can use these adapters to create a bridged output, into which you can insert a third 25-pair cable, and from it derive the pair(s) you need.
Many of us dinosaurs have 25-pair adapters in stock, from the previous century.
