Why does the HD2 & HD4 MBX 5G has four antenna connection per modem?
Because its 4x4 MIMO
https://www.qualcomm.com/news/onq/2019/06/20/how-5g-massive-mimo-transforms-your-mobile-experiences
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is there any solution that lets one combine multiple antenna connections at the peplink unit into fewer wires, then split out at the other end (where antennas are deployed)?
We supply custom RF solutions that are designed specifically for situations where you do not want lots of individual antennas; the solutions include consideration of RF power levels & RF signal isolation & frequencies. A solution can easily cost several thousands of dollars for the MBX HD4 Cat18/Cat20. We have created customised solutions for various industries, including construction, maritime, emergency services, and others. We also have supplied these for customers upgrading from the older HD2s to new MBX HD4s.
Even though Peplink has the new Mobility & Maritime antennas (& we do like the range), these do not solve the issue of limited cable pathways or mounting locations or where cable runs have to exceed more than 7 meters. We often run cables over 15 meters, so we re-design the whole RF solution to take that into account.
A word of caution, do not use RF Splitters; you will cause more issues than you solve.
Due to the commercial value in the solutions & designs, we can not share or publish them here, though you can contact us to get a custom solution if required, please note these are sometimes more expensive than the router itself as they are customised.
Happy to Help,
Marcus 
Dear @mystery,
You can choose Mobility 40G or Maritime 40G, so one antenna per modem of 5G a device.