they are very different stones (and made by different people)... the 5k is a splash and go stone... its not the fastest cutter, but it is the best stone i have in my lineup for cleaning up an edge (reducing the burr/ removing the wire edge). The 4k is a different beast all together... its fast, agressive, has great feedback, and leaves a nice edge. It can be a bit more difficult to get a super clean edge off of the 4k (just off of the stone... not including felt, cork, newspaper, etc) because of how fast it cuts.
I also have a 6k that is splash and go... its much faster than the 5k, so i use that sometimes to finish.