IIRC, 49 in Japanese wordplay can be read as Yon-ku (which sounds like Yonko). Maybe
@Giggio can explain it better to you.
Also, I realized later that not only it refers to the number 4 (often associated with Death), but it does also to the number 9 (like the 9 shadows and Zoro's 9-sword style as Asura), so 1049 had been predicted to be pivotal in setting up ZKK.