2018 should be considered 3rd gen based on the groups that debuted that year. The biggest 2018 rookies were Stray Kids, Ateez, Fromis_9, Gidle, Loona and Izone. All of them except maybe Gidle have much more in common with other 3rd gen groups than 4th gen groups.
Starting with the girl groups, 3rd gen girl groups were known for their cute concepts, large number of members, fun and catchy choreographies and mainly targeting Asian countries like Japan. While 3rd gen ggs were more dance focused than 2nd gen they still had powerful main vocals and high note compilations were standard. Fromis9, Loona and Izone all share the majority of these traits with groups like Twice, ioi, WJSN, Gfriend etc.. Fromis9 and Izone also debuted from Mnet idol survival shows which dominated a large part of the 3rd gen. On the other hand 4th gen groups like Itzy, Everglow and Aespa have girl crush concepts, are dance and performance focused, target an international audience and have a smaller number of members.
Fromis9 debuted with To Heart, a song innocent and cute enough to rival early Gfriend. They also debuted from an Mnet survival show that aired in 2017. Their main focus is Korea, not the international market.
Loona started to drop solos all the way back in 2016 and would have debuted in 2017 if their debut hadn’t been pushed back. While they’ve later swapped to a girl crush concept (like many other 3rd gen groups) they debuted with Hi High, an energetic and cute song. Most of their predebut promotion also leaned more towards the cute side, especially their sub units. Plus 12 members are close to unheard of among 4th gen girl groups.
Izone is a Produce group with 12 members targeting Japan. If that doesn’t say 3rd gen what does? Their concept is elegant but again much closer to being labelled as cute than girl crush.
Now for boy groups. Boy groups have been floundering the last few years not really finding anything that sticks and the difference between generations is less obvious. Still both SKZ and Ateez seem to have been created with 2015 BTS as a mold. Powerful dancers and performers with a strong rap-line and a hiphop concept. New 4th boy groups like Txt, Treasure and Enhypen all have much softer concepts. Txt and Treasure have fresh, youthful and even cute concepts. Enhypen also has a less aggressive concept compared to Atz and Skz and have a more pronounced sweet boyish side. Rap is also less of a focus point. Txt and Enhypen don’t really rap and even Treasure, a Yg group, is more vocal focused.
My point is that most 2018 groups share more traits with 3rd gen groups than 4th gen.