It’s worth considering the music video too. In general, it was a lot of barely there moodiness, informed by Tumblr aesthetics, with a shapeshifting aspect ratio and an art film take on the butterfly effect in its narrative. Once again, this was in stark, stark contrast to many of their old videos, which were generally ‘box music videos’, a style popularised by S.M. Entertainment, where the idols would dance in a brightly-lit, flashy and colourful box set. Reinvention didn’t have to be loud, they proved.

For those who looked closely, perhaps the reinvention was more obvious than it seemed. 2015 was the year that London-based production duo LDN Noise started lending their hands to k-pop, particularly for artists in S.M. Entertainment. Their eye for the UK music scene brought a whole new perspective on the sounds that dominated k-pop. That year, SHINee (commonly considered as f(x)’s sibling group) released ‘View’, perhaps the first hint of the house influences to come with f(x)’s impending comeback later in the year.

LDN Noise contribute three songs to the album and are almost immediately recognisable as such. There’s one in each third of the album, so you get a little reminder of f(x)’s new influences and sounds throughout the album’s listen. The three songs are also roughly in order of sheer insanity, going from the subdued ‘4 Walls’ to the 90s piano house anthem ‘Rude Love’ to the brass-laden, Latin pop tribute ‘Papi’.

The dance vibe of the title track is not a simple one-off on the album as ‘Rude Love’ proves. LDN Noise channel 90s house, particularly in the opening bouncy piano riff that was so distinctive about much of that genre. Consider the similar treatment of the pianos in the chorus of Madonna’s ‘Vogue’ or, for a more recent example, Katy Perry’s own piano house tribute, ‘Walking On Air’. It feels particularly interesting showing up on a k-pop album, since it’s not a genre that was really ever present in Korea in the 90s. It feels quite 90s, but completely modern at the same time, by virtue of new Western influences.

‘Papi’ mines Europop and 00s dance music even more heavily, in its thumping four-to-the-floor beat and high-velocity synth riff. It feels very suited to the club floor particularly with its shameless chorus cry of “Papi, let’s go! Cause I kinda like it” which explodes with unrestrained ridiculousness. Hell, there’s even a Latin style breakdown at some point during the song just to push the song to its very extreme. It’s very difficult to imagine any other group but f(x) releasing this sort of song, a tribute to both this album’s consistency of sound and f(x)’s unrelenting personality.

More veteran names do appear though alongside LDN Noise on the album. Rapper Zico (of Block B) lends his rapping expertise to ‘Traveler’, a melancholy 80s track and the slowest moment on the album. It is however one of the most tender moments as well and show that f(x) can revitalise ballads into something worth listening.

S.M. in-house songwriter Kenzie contributes to the album, as does songwriting collective Jam Factory. Andreas Öberg, a Swedish guitarist and prolific k-pop songwriter, is responsible for ‘Glitter’, the most sugary moment on the album. That said, its chorus drops with feverish intensity and some brilliant harmonies (and the bridge is a moment of pop joy). Norwegian collective, Dsign Music, provide both ‘Déjà Vu’ and ‘X’. Those two songs’ Scandinavian influences are quite clear, particularly on ‘Déjà Vu’ which feels like an Annie song in its brash but sophisticated take on synthpop. The ubiquity of Scandinavian songwriters and producers is as common in k-pop as it has been in Western pop, which is dominated by the likes of Max Martin and Shellback.

4 Walls against the odds feels like a cohesive collection of pop songs. You wouldn’t see a post-Disclosure garage-pop song next to frosty Scandipop on a Western album (probably), but k-pop’s penchant for eclecticism makes the two fit together very naturally.

In general, k-pop often feels like warped, twisted versions of Western trends. The songs mine an era or a genre: but will have a weird production quirk, or a harmony that you wouldn’t usually expect, or something along those lines. Songs that would simply exist on a Western artist’s album are given choreography and promoted endlessly on TV shows in Korea. Imagine Disclosure’s ‘Latch’ being performed live at an awards show as Sam Smith busts some moves. Yet, that’s basically what is happening in k-pop all of the time. ‘4 Walls’ was rendered with a simple but lusciously elegant routine that referenced voguing and waacking. ‘Diamond’, a quirky almost trap-inspired tune gets a perfectly synchronised, attitude-filled dance when performed live that is worth noting since it’s much less feminine than a lot of other girl groups’ dances. f(x) has often had more gender-neutral and intricate dances throughout their career, yet another one of the interesting things that have made them so unique.

Sulli was never the strongest vocalist in f(x). She left to focus on her acting career, and perhaps the music industry was never for her. The parts she had in f(x)’s songs was minimal, usually taking over parts of the bridge and rogue lines in the choruses. Although she had a nice vocal colour, providing a shimmering air to their discography, she was ultimately vital in the group on account of her personality rather than her talent.

That said, you can see how f(x)’s approach to vocals has changed in her absence. Luna still leads pretty much every song (and with good reason as she’s one of the strongest vocalists in k-pop), but she takes over a few of Sulli’s lighter harmonies. More than ever, Amber is given more singing parts on this album. Her warm alto tones materialise (quite notably on ‘Rude Love’) quite joyously, and it’s to the extent that her singing is often more enjoyable than her rapping. At no point does the mixing up of vocals feel like a compromise however: in all ways, it is a feature of 4 Walls, and should be celebrated as such.

Perhaps most curious of all are the two final tracks on the album, ‘Cash Me Out’ and ‘When I’m Alone’.

‘Cash Me Out’ is perhaps the biggest deviation from the rest of the album’s fusion of frosty indiepop subgenres, instead opting for a big continental EDM sound. It does threaten to sound like the most dated cut on the whole album, but it does at least fit in amongst the other dance-influenced tracks. It’s of particular interest because it actually used to be a song by Zara Larsson of the same name, long before she blew up with ‘Lush Life’.

The closer of the album, ‘When I’m Alone’ is an 80s synthpop anthem. If that rings any bells, and those bells were Carly Rae Jepsen’s E•MO•TION, you’re along the right lines. ‘When I’m Alone’ was written by Carly Rae Jepsen and Bonnie McKee as part of the E•MO•TION recording sessions.

So I worked with this amazing artist called Bonnie McKee, who I worked on Kiss with, and we got in the room and we were both in a really weird mood and we wrote a song about, basically being alone … Trying to find a delicate way to say this. It rhymes with contemplation…”

‘When I’m Alone’, the enigmatic song written about masturbation was toned down lyrically and ended up being the closer on an album that feels very much like a parallel of Carly’s.