It shouldn't be a surprise, really. VW group has been cutting costs on all new designs in the last few years. The big e-tron was one of the newest models before the cost cutting started.
Anything Audi MQB has always been closer to the VW equivalent than it has been to the bigger Audi MLB cars. MEB is the same thing, it's the shared VW platform that is mostly VW level with Audi design. The old A3 (8V) was great in build quality, but so was the Golf of the same generation.
Audi's new design language involves more fancy materials and designs that are more sensitive to build quality issues. The old designs were soft plastic all over, which feels premium and doesn't creak. New designs have big touchscreens, sections of gloss black and have less soft plastic to save weight and cut costs. In the MLB cars (A6/Q7/e-tron and up) they made it work but they're expensive to build. The old MLB cars (A4/A5/Q5) are still the classic solid Audi design and build.
The Q4 is in line with the Q3 and new A3 when it comes to interior class, and both the Q3 and A3 are a step down from similarly sized Audi's from the generation before. Less quiet, less refined interiors and a more VW-like feel.
The big e-tron is in a completely different league and you really can't compare the two. Remember that next year the Q6 e-tron will slot in right in the middle between the Q4 e-tron and fat e-tron in class. It'll likely have a new generation of interior (that will probably carry over to the Q5 non-EV sibling).
So TLDR, the interior in the Q4 e-tron is most easily compared to the Q3 or new A3, and is definitely a step down in perceived quality from the old A3 (8V).