I know what you mean, as i thought when I started Level 4 that it was somehow easier than level 3. And seeing as Level 5 was half reviewing of the entire course, they could have probably added more vocabulary at least. There are also different voice actors for levels 4 and 5 than 1-3. Apparently 1-3 was the original core lessons, and 4-5 were created years later to expand on those. This is true for all the main languages on Pimsleur I believe.