I get exactly what you mean by the prose. I'm also writing a book with 4 POVs and I'm careful to keep their perspectives distinct. For example, one character is keen to use figurative language, while another is very direct in his expressions. When you write multiple POVs, you want to be certain to relate the chapter from your character's POV, rather than from your POV as an author.
However, I'm also writing this book in 1st person, where I think it's more important to keep these distinctions clear. I think you might be alright in 3rd person, because you're not directly relating your characters' perspectives.
If you're determined to work on here, these are some of the things I think about for my own characters:
- How do they express things? Are they more figurative or more literal?
- Do they pay careful attention to details or do they look at the big picture in situations? E.g. some of my characters will point out aspects of their setting, while others will ignore it completely because it doesn't occur to them to do so.
- How do these characters perceive the people around them? If one character loves Amelia but another character hates her, they would likely use different terms to describe her.
- Overall (the "big picture" question): how do this character's experiences affect their perception of the world? For example, some of my characters are more educated than others, so they can perceive the world in a more informed manner.
- I think a lot of the points Buffy brought up are also good. Not just for dialogue, but for how the rest of the chapter is written too. However, this is likely more important in 1st person, where each chapter is written in an individual character's voice. I'm not sure how important it is in 3rd person aside from dialogue.
TL;DR: I don't think it's that important in third person to differentiate POV, but I could be wrong about that. If I am, these are some of the techniques I use to differentiate POVs in my 1st person novel.