LogFAQs > #961539904

Kaeya and Rosaria are both good in very similar ways. They're basically interchangeable. If you haven't leveled either and aren't planning to use this character as your main DPS, I'd say work with Rosaria, but Kaeya's possibly better as a main, and you've already got him in use, right? Or do what HDT suggests and use both.

What's your current team again? Razor, Ning, and Barbara it suggests above? I wouldn't necessarily recommend both Razor AND Ning since they're both "greedy" characters that want on-field time. But the most important rule is play what you want to play, so don't put too much stock in that. Razor wants a cryo. Ning likes a second geo, though it's less important.

If I were to rank the 4-stars for non-main DPS purposes, under the assumption we're talking about LATE GAME purposes, and not for leveling:

1. Xingqiu
2. Bennett (most people would put Bennett at -83 on this list, because he'd be so high he'd pass through the negative numbers)
3. Sucrose
4. Diona
5. Xiangling (who is also a main DPS)

With honorable mention to mostly everyone else. There's almost no character I would say is universally useless, even if some have very narrow uses.