@H0R0, it's still swinging, last chapter (16), Yukari says that Seiji and Risa are also having sex just as Yukari and Komiyama do (maybe not the same time) but the swinging still happens, just off-screen. As per chapter 13, they don't only swing in the same place together (as 4) anymore but also in different places in pairs (although I'm sure they may occasionally swing together as 4).
Also, I think we'll start to see less and less of Yukari and the gang for a few chapters at the least, as she mentions on page 20 that she and Seiji are going overseas for a while for work, and in the mean time, she (cleaner lady) has to get her partner's ok with swinging if she wants to fuck Seiji (as per last chapter, Yukari is only ok with it if it's swinging/swapping). Or something like that...just had a quick skim...I'm feeling less and less interested in this series, next chapter may be the last one I buy depending on how it goes.