Depends on who you talk to! 4:1 - 5:1, what's one little scoop of sand amongst friends?
Five-to-one will certainly work but in my experience if the area is large and there will be a lot of movement on the raw surface after it is set-up (prior to tiling) then 5:1 is a little too sandy for my liking.
Four-to-one will shave the same as five-to-one and be a little stronger and durable during the tile installation.
It's your choice.
Besides, that information was written with the DIY'er in mind and assumes a one-time first-time project.