Hello Paul Sheraton,
Welcome to Microsoft Community.
After reading everything you've posted here in its entirety, it's clear to me that your DELL monitor should also be able to display at 60Hz, as the Surface Laptop 4 is capable of doing. Allow me to share some parameters here:
Supported external display resolution and refresh rates for Surface - Microsoft Support
Therefore, I would consider whether the problem is due to the connection between the Surface Laptop 4 and the DELL display, and yes, I would speculate that the problem may be related to the cable, adapter, or docking station being used. But I don't think they're in a broken state, because with the same cable, adapter, or docking station, you can get a 60Hz 4K external screen in an Air Macbook perfectly.
I think the cable, adapter, or docking station you're using may not be compatible with the Surface Laptop 4. Visit the following support links and I recommend that you check the information in Solution 5 and Solution 6 as a priority:
Troubleshoot connecting Surface to an external display - Microsoft Support ****
If you want to test for this problem, you may need to replace other cables or adapters.
And, as an additional suggestion, I might choose to use a cable that is fully compatible with Surface devices, or an extension dock:
Accessories Designed for Surface (microsoft.com) ****
Buy Microsoft Surface Dock 2 – Microsoft Surface [fdcd-9882-a7c-e9e7]
Buy Surface Thunderbolt™ 4 Dock (Ports, Compatibility, Price) - Microsoft Store
These accessories have been developed specifically for Surface devices and offer unparalleled compatibility.
However, if you still want to try to troubleshoot or resolve the issue without using a third party cable, adapter, or dock, you may wish to report the issue to the cable, adapter, or dock supplier.
Best Regards,
Mitchell | Microsoft Community Support Specialist