Aloha Harry,
Here is a link to the company that is the source to many of the suppliers in the US:
https://www.bmh-ltd.com/
Their website doesn't mention TR4 panels but I have bought pieces for my TR3 that they had manufactured so I assume they make them for the TR4. I have bought panels from the big three and most have been fabricated by British Motor Heritage. Their parts usually have their label on them.
A few years ago, a truck collided with my TR3 and it needed a new front cowl and right front fender. The auto body shop got BMH parts through Moss. The fit was generally good, but did require a little work to get caps correct, fill in around the head light buckets and make the grille reveals uniform. I don't think this is unusual for a bolt together auto body, particularly one that is about fifty years old. The auto body guys were pleased with the quality of the parts.
In another instance I ordered a rear quarter panel from TRF. They contacted me to tell me that the one they offered (didn't name their source) would require more than average effort to fit in the car. This panel has several curved surfaces and is mostly welded in. I was using the piece as a source for a patch panel so I told them I still wanted it. It worked fine for my purpose. As a complete replacement it would have needed hammer and dolly work to fit. My point is, TRF was very proactive to make sure I would be satisfied.
The advantage of a new panel is that it is rust free but on the other hand it tend to cost more. Used panels may have some rust or pitting, but the lower cost may be off set by the time and effort to repair them. In some cases only used parts are the only option however.