The Thinkpad T4x0 series has two batteries as well. I had a T450s at my previous job and I think I'd get 6-7 hours out of it (usual workload of ssh, vim, 50-100 chrome tabs, slack, google play music all running).
Most of the issues were sorted out by kernel updates, and a BIOS update I performed recently.
The only thing that Arch clearly improved was the Bluetooth experience, which was very unstable on Ubuntu (even after the upgrage to 17.10). But even that is probably down to the newer kernel in Arch.
You can do better! I get near 20h some times longer with about the same work loads. Make sure you have tlp and use the settings suggested by power top. There are also a few options to pass to the i915 module that increase power savings.
