Cluster sizes are in bytes, not k.
Cluster size determines the allocation unit, not the size or reads and writes. And anyway, writing 4k is negligible as compared to accessing them.
As for the best cluster size with Ntfs, the answer is: the default one.
It's the size that allows defragmenting Ntfs with Xp, for instance. And Ntfs doesn't accelerate with bigger clusters, as opposed to Fat32.
You are correct, my brain must be getting mushy - time for my nap.