Regarding the video quality, I think it's more likely that HW4 video output downrezzed (aka down-sampled aka decimated) to HW3 will be of somewhat higher quality. I haven't seen any direct comparisons but:
- There are theoretical and practical benefits from the original image being taken at a higher resolution i.e. spatial sampling frequency. I wouldn't put too much on that without testing and knowing more about the level of anti-aliasing measures taken in both hardware setups.
- Aside from pixel resolution, I believe the new cameras can operate at a higher frame rate. I don't know if they are simply being run at the HW3 rate now, or if the video is being processed to normalize the rate.
- Perhaps the larger effect, in the Tesla HW3-HW4 case, is that the newer-generation cameras supposedly do a better job of suppressing sensor overload artifacts from bright sun in the frame. It's also likely that the low-light performance and noise floor is superior.
I agree that the video down-sampling is not the whole story regarding Elon's comment. Aside from the question of video input quality, I think he means that clips from both HW3 and HW4 are used to train the same model, and then both in-car computers are running the same inference model. Not taking any real advantage of the more powerful computer in the HW4 vehicles.
While in general, there can be a loss of performance in having one computer type run another's native code using an emulation layer, I don't think that itself is a significant problem here. I would be shocked if the HW4 computer had not been engineered to run HW3 code directly at full performance; I think it was essentially a mandatory design requirement for the rollout of functional HW4 vehicles in 2023. Perhaps "compatibility mode" would have been a clearer term.
So I don't think the "emulation" represents a loss of quality in video or in compute - more likely some modest actual gain in the former. Elon's comment should mostly be taken to mean that HW4 hasn't achieved its full potential, because they haven't yet deployed a version customized for it.