I've always had much better luck promoting the first book in a series rather than later ones.
This sounds like a Push vs Pull sales strategy problem. And I think it's super common with self-publishers.
Most advertising works by shouting at the customer to buy our stuff. This is why we get louder and more flamboyant ads designed to get people's attention. But people are already inundated with ads. They only have so much attention to spare. This is the Push strategy.
Buy! Buy! Buy!The Pull strategy (using branding) just lets people know who you are, what you're about and where to find your books. It's informational as opposed to a call for action. The trick for this to work is to be super visible. Instead of running one ad, or maybe a couple of promos on the usual mailing list sites (Push), you use social media - a lot. It takes time, but the readers will find you. So, talking about that fourth in the series is another opportunity to tell a story about your brand - more posts, more engagement, more fans. And for platforms like You-Tube, those posts are always going to be there for new fans to discover. So, instead of
one-and-done, do the work, and then reuse it as much as possible - keep it working for you.
But you don't ask people to buy your stuff in these posts, that doesn't work, and this is why most self-publishers fail at social media. They think it works like Push advertising. It doesn't. Most of your posts should be about other stuff, not your books per se. But through your brand personality (what you talk about and how you talk about it), readers will figure out who you are for themselves. The ones that like what they're hearing will check out your books - and everything else you do. Some will become fans. Fans are promotional vectors, massively expanding word of mouth recommendations, reviews, engagement, etc. Casual readers don't do any of that. Yeah, you got the sale, but then they just move on to the next thing.
The old days were about Push strategies. The algorithms are about Pushing titles, especially the sponsored ones.
With market segmentation, we can use Pull strategies, which for us, is way more effective; and offers the upside of making the algorithm irrelevant.