Maybe there still are some dependency on the audio how the quality falls out within the two models.

If one of the stems does not fall out so good e.g. the Bass stem.
It could be worth it making two sets of stems, just to check.
One with the new 4-stem model and another with the 6-stem model and then compare them.
(it is easy to compare just listen or otherwise drag them all over to the DAW and compare).

I just made some simple tests, and with one of the audio tracks, an old classic recording on mp3 picked up from YT the Bass stem was surprisingly better with the older 6-stem model than with the new 4-stem model (at least from what I could hear and see). Maybe it is very rare.
Folsome Prison Blues - Johnny Cash