Thematically different (lmao!!!) is not about core gameplay, it's about the "thematic elements."
L4D is a co-op survival horror game, set in nighttime locations (sans The Passing, which was built during L4D2). It focused a lot more on tension and survival, while having mild comedic moments between characters.
L4D2 threw out most horror elements (sans zombies) for a comedic action zombie shooter. They are nothing alike in terms of themes. I can't stand how goofy and satirical L4D2 is compared to the original, it completely ruined what was so innovative about the series (making a co-op experience that was, at times, actually kind of scary and tense).
The gameplay is basically the same, they didn't really improve it, so much as just kind of added to what was already there with more weapons. The extra variations and maps are a cool point, and give the game way more replayability, but it's still super goofy and slapstick.
Evolve was garbage, but has nothing to do with L4D outside of the developers (who I'd hope learned from their mistakes). As for B4B, I never claimed they weren't using L4D for their own publicity, but at least they are keeping to the themes they originally set. If you look at how Valve influenced the changes to the original L4D, and at what came from L4D2, you realize just how much creative control was taken away from the team and how much their intent was lost between games. B4B also didn't come out a year after L4D, unlike L4D2.