E Class doing nearly 70k/year.
5 Series doing 50K+/year.
A6 only doing hardly anymore than GS, although the 2nd body variant called A7 adds another useful 8K/year.
The main reason 4GS isn't doing as well is probably due to "conservative" or "sterile" exterior styling, and in particular, the very conservative dash and central console inside.
IMO, 4GS sales would get a lift if there was a bit more oomph in the styling.
However personally, I prefer the 4GS' neat, conservative, sterile, and safe styling over my wife's 3IS250 which has some odd "lines' that I do not like, like the longish nose, the curvature on the waist line beneath the wing mirrors, the way the waist line almost drops down at the rear window above the rear door, and too sharp a trailing edge of the boot, but yes, the 3IS does have more oomph to the styling outside and inside.
I will get my 2015 4GS very soon.
In the past, none of the Lexi that I have purchased have been perfect. In life, nothing is perfect.
In 1999, I purchased the 3ES, and I had to put up with its bland 1980's style tail lights, which were not "kicked up".
In 2010, I purchased the 3GS350 and accepted the 4 bug eyes which I don't really like.
Now 2015, I will get the 4GS, and live with the dash and central console - what I consider to be the 4GS' biggest Achilles heel.
However, in all three Lexus models, the rest of the car is superb.
Presently, the current E Class and 5 Series don't look super flash outside, or inside either.
As for sales volume?
I use WindowsPhone 8.1 for Microsoft Office mobile eg Word and Excel editing, and I don't care that Microsoft WindowsPhone 8.1 has less than 10% market share to Google's Android, and iPhone.
I believe we must do what suits our individual tastes best...