The rest of Chapter 4 can be found here: https://myreadingmanga.info/suyohara-this-guy-eng/4/
If you want a summary (there's only one partial sex scene right after the pages posted here end and it gets interrupted by a flashback), the older brother and the doctor knew eachother in highschool. Back then, the doctor helped the brother skip a practice (soccer? It's not specified) and they become somewhat friendly, though the brother keeps teasing the doctor about potentially being gay. The brother tells the doctor he needs help studying; the doctor says he'll do it on one condition. The condition is that the brother has to rent [straight] porn movies for them to watch. They watch the first one at the doctor's house while his parents are gone. There are several scenes of the film shown on the tv; there's one or two decent dick pics but there's a woman in the scene, so it's pretty meh. While they watch, the doctor gets a boner in his shorts and the brother starts side-eyeing it, but the doctor catches him and flips the "lolol ur gay" joke back on him. The brother admits that he's gay. They kiss. They watch the porn man fucking the porn woman [ew] doggy-style and the brother says he wants to do that with the doctor. That's where the chapter ends; the scene picks up immediately with chapter 5.