The rank of a kata is set according to the votes of the ones who solved in when it was in beta. Then it cannot be changed. Old katas tend to be overranked, it's rather the contrary on newer ones. Moreover, consider kyus are shared across languages and there may be great variation of difficulty between them. Many discourse pages are full of comments saying the kata should be one kyu above or below. They are useless.

Nah I struggled about 2 hours with this. But I solved other 4 kyu katas in 5 minutes. Sometimes its easier, individual thing.

I agree. It's probably a 5.

Maybe it could be a 5. I really struggle on 4s and I had to think about this. But basically it doesn't require much knowledge, you just need to understand the logic that has to be applied.

Older katas tend to be easier.

I've been struggling with this one all day, can't figure out a good algorithm to get it working. I think 4 kyu is about right. Certainly not 6.

Idk you are definitely getting better cause i struggled big time with this. But mainly because I went through so many different solutions, wasting time and stuff; the trick here was i expected everything to be sorted.
In the end, pulled out the good ol' sort algorithms, and finished it in 30mins. I should say my solutions are always super unique to everyone elses.