The Highschool DxD Hero (or High School DxD Hero) anime was apparently a surprise to some of the cast members. After several years of waiting between seasons, Highschool DxD Season 4 continued the story of the light novel series by author Ichiei Ishibumi and illustrator Miayama-Zero. Fans are just happy to see Issei and Rias again on their TV screens, but so are the voice actor for the characters.
When Yuki Kaji (voice of Issei Hyoudou) first heard about Highschool DxD Season 4 being produced, his first thought was that Hero had an "amazing title" since "H!" and "Ero" translates to "perverted" and "erotic." Yoko Hikas (voice of Rias Gremory) also had a "surprised reaction" when she first heard about it.
"When I started working on the first season, I didn't think it would continue on that long, so I am really happy!" she said, according to a translation by Manga Tokyo. "It is indeed a series that is watched and loved by many people. The power of ero is strong!"
Maaya Uchida (voice of Irina Shidou) noted that Highschool DxD Season 4 "took some time to come after the third ended! And that means the expectations of the fans would also be considerably higher." Rikiya Koyama (voice of Azazel) was also surprised since "normally this kind of series [will] end after a single season and they leave everything just where it was without a care in the world." Notably, anime studio Passione retconned some of the errors of the third season.
The cast was similarly of one mind when they were asked to describe the "charm of High School DxD in one word." It's all about the "oppai," which literally translates to "boobies" in English. However, some members of the cast felt that the anime goes beyond the ecchi fan service.
"The DxDness. This word is slowly coming out during recording," explained Ai Kakuma (voice of Rossweisse). "Everybody who watches the series feels it somewhere, the fun it is to record this series. It's the balance of the hotness and seriousness and pervertedness or rather the miraculous exquisite words."
Rikiya believes that if "they push that again, the fourth season will be continued." Although he didn't say it directly, the actor probably means High School DxD Season 5 since the current anime will end in the middle of the Heroic Oppai Dragon story arc.
The Highschool DxD light novel series recently ended in 2018 with Volume 25 being the finale, but the author is creating a direct sequel called Shin Highschool DxD. Thankfully, that means there are plenty of source material for creating new anime seasons.