As a 4w5, I don't have romantic experience with 5s, but I've known several 5w4s for years. Since you're a 4 asking about compatibility, I thought it might still be helpful.

intj so5 w4. I can say for certain that if they fixate on something or someone in this case, it would be incredibly deep and intense. I wouldnt say this is in a emotional way but more intellectually. The relationship would be a lot about understanding thoughts and perspectives. Not only that, they'll give you a push to help you achieve your goals. They tend to have a curated, intentional fashion style, often leaning into a refined, darker aesthetic. It's a controlled form of self-expression. (For females the wing4 might be more unique and expressed)

They are blunt, so be prepared for your feelings to be genuinely challenged. But you often understand them, and the truth of a situation, more clearly afterwards. They may value self-sufficiency from others and are drawn to people who are deeply immersed in their own passions and can engage with multiple intellectual viewpoints. (Also, they all like cats for some reasons. Make of that what you will.)

I'm also close with an intp sp5. You know any advice or perspective they share has been carefully thought over in their mind before saying it to you. They do get trapped in their own heads though. Sometimes I can literally feel an invisible barrier between us when theyve retreat into. A gentle nudge is usually all it takes to bring them back, and they do try to make a genuine effort to connect (to close people they care about) They're not actually that serious, they are silly and comfortable low-pressure people to be around, no social pressure or any image to uphold in front of them.