You cannot stay with Sage in Episode 4 without becoming her fuck buddy. You can turn down the chance for sex with her in Episode 4 if you want, but you will automatically fuck her on Tybalt's bed in Episode 5, and outside the HOT house in Episode 7 (though you can shorten the scene if you have sex with someone like Riona first).
Fortunately, you don't need to stay with Sage to start the potential Riona romance. All you miss out on is the scene with Riona listening to music in the garden (which is admittedly a nice scene). You can still make out with her at the DIK party, give her money in Episode 5 (though not have sex with her unless you're a restaurant member), and hook up with her (and potentially Camila as well) in Episode 7. And all of that is doable with CHICK affinity.
If you do hook up with Riona in Episode 7 (with or without Cammy), she'll send the MC a text at the end of Episode 7, and you'll be able to track down her stalker in Episode 8. If you've already got your screenshots of Riona in the garden, my advice to you is to redo the run without staying with Sage and hold out for Riona in Episode 7.
