Big yikes on the end of ch4. MC probably feeling pressure from meeting the ex and took "couldn't be happier" a little too seriously. Hope it doesn't get gross from here...
In case you're wondering and don't mind spoilers, Takagai(the kohai) tells Shiina(the MC) no in no uncertain terms and Shiina backs off, but this ends up causing things to be awkward between them until halfway through the final chapter.
If you'd like to see it for yourself, go check out the series on Renta! It only costs $2 to rent the remaining two chapters and see the rest of the story. Even the free preview for Chapter 5 resolves the cliffhanger.
I'm a bit tired of the drama on the discussion forum surrounding this chapter. I hope that once more people see how the rest of the story plays out, they'll calm down a bit.