Well sort of, we have a pro 4 freehub on a 142 width can I use it on a pro 4 148 hub? Just a case is swooping over the hubs and using wider end cap or not?

Yes it’s the end caps that change the width.

I would try swapping over the freehub but keeping the endcap / hubs together.

Should all be the same… Freehub and caps. Boost hubs are different to non boost

Difference is in the hub body so endcap and freehubs are interchangeable. Only exception is if the 142 has been boosted. Note different end caps for different freehub designs.

Yes it’s the end caps that change the width

This is wrong unless hub has been busy converted.