User blog comment:HollowIchigo58/High School DxD Season 4 Announced/@comment-27083298-20161024104058/@comment-9595671-20161027081727

Yeah, the only ecchi light novel anime I know of that has a fourth season is Familiar of Zero. In fact I don't think I've ever seen any light novel anime adaption reach a fifth season. I don't count slayers because three of the seasons are anime exclusives. Does anyone know of such a series?

And even less get an anime adaption that adapts the entire series, I may be wrong about this example cause I'm not well versed in the series, but I think Shakugan no Shana's anime is one such adaption.