I would recommend the Michel Thomas Method. The website can be found here. The Advanced course may be beyond Pimsleur IV, however I am not totally sure. I would check out reviews of the difficulties. The method is also a bit different so it may also offer another angle of approach for the learning the language, which will diversify your ability and help.
The Michel Thomas Method is unique. It uses a translation and memory based approach. You can technically use it in your car but he does not recommend it due to the distraction.
When it comes to learning German it doesn’t get easier than the Michel
Thomas Method. The Michel Thomas method eases you into the German
language by breaking it into its component parts. You will then be
able to easily form your own to say what you want in German! Go from
absolute beginner to confident speaker—without books, homework or
Another great resource: Langsam gesprochene Nachrichten. I know it is technically from the DW, but if you haven't tried it, it is great. Many of my friends use it to improve their listening comprehension. That you can definitely use in the car. Plus it comes in Podcast and you won't ever run out of content because it is a news program, continually being updated.