Katanas definitely have more parry frames than stated here. I tested it with the Chaos Blade on the Black Knight with the Greataxe right after the Champion Gundyr bonfire. I recorded as I was playing, parrying him over and over. Now as we all should know, the parry frames start when your stamina bar goes down, and counting the frames afterwards revealed that I got successful parries more than 4 frames after.
e.g. taking the stamina loss frame as frame 1, then the frame where the parry sparks came was on frame 6.
Another time it was on frame 9.
If you take the frame that fp is consumed as frame 1, then the frame stamina is lost (thus the parry frames starting) is on frame 9, exactly the same as fist weapons.
In this case, my previous tests landed successful parries on frames 14 and 17 respectively. Thus it seems to me like they share their frame data with fist weapons, the only uncertainty is that they might end 1 frame earlier or a couple of frames later.