This may seem silly but I wanted to be sure.
There is an asset pack of models that has a supported version of 5.0 and 5.1. If I purchased them, installed UE5 I could just export the model as a raw 3d model file type, then import into my UE4 project with no consequences (beyond maybe loss of detail, skeleton rigging etc)?
It’s mostly the act of taking something sold for UE5 and using in an UE4 project.
No, it doesn’t!
.uasset files are not compatible with previous versions of the engine and you can’t use that process to put assets in older projects, unless the creator used an old engine version to create them.
You are free to export them and re-import them in an older version but you will have to recreate all the materials nodes manually and copy/paste nodes, and it could not work, because many nodes changed from UE4 to UE5 and UE4 might crash when you paste!
While the answer by @rokenrock above might be true in some cases, it absolutely not to be taken for granted. For instance, there has been a changes in the way the engine handles models as recently as 4.25 or so - meaning that .uasset models authored with a newer version would simply not show up in a project create with an earlier version of the engine.
In short : while it is of course possible and totally fair to mark a an asset originally built with 4.2x as being compatible with 5.x (because it is), do not expect an asset made with 5.x to work with the previous generation of the engine.