I'm playing 1.5 and supposed to friend Marlon, should be a cut away scene after Four hearts. I have full hearts with Marlon but haven't triggers the cutaway scene... Yet!
Okay, yes, we understand that you're playing 1.5. The problem here is that Marlon doesn't do this for you
EXCEPT in Stardew Valley Expanded (aka SVE). That's a mod. If you're just playing Stardew Valley 1.5.4, Marlon is never going to do this for you. Gunther, at the museum, is the guy that will do it and only after you've donated enough things to the museum.
If you
ARE playing SVE, this is an issue with the mod, and not the game. Problems with mods have one section, and problems with the game itself go in this section. That's why we're trying to narrow some things down, so you can get the proper help.
So, please, are you using SVE? If not, keep donating to the museum. If you are, please see this link:
https://forums.stardewvalley.net/threads/stardew-valley-expanded-rusty-key.4869/#post-29292 or at the very least this one:
https://forums.stardewvalley.net/forums/mod-support.29/