Yeah, I think most fans assumed Book Four would be the last, at least in the LoK series. I think what fans were really worried about was whether or not we'd even get a Book Four at all. As long as the series is ended properly, with the characters and storylines getting the appropriate closure they deserve, I don't think we'll have any issues.
As for them doing another series. With this kind of creative team, I honestly wouldn't say it was out of the realm of possibility. Originally it was thought they'd never do anything past ATLA, so whether or not they choose to do another series I believe is still a possibility.
Personally I wouldn't mind them doing a prequel series, possibly following the adventures of an Avatar set before ATLA. But when it comes to a show like this, I'd only want to see another series so long as the original creators are up for it. If they feel like they want to move on, I think that's completely reasonable and wouldn't really be upset about. But if they feel there's another story they want to tell in this amazing world they've created, I'd love to see what they can come up with.