4th sub- module effect rerolls

I have a mythic + harmony. I wanted to roll 4th sub effect DW quantity. Spent about 400k rerolls. No luck. I thought ok I will wait to lab 2 bans and some more rerolls and try again in 1 week. After 1 week I spent 500k more and no luck again, even with 2 bans. I am wondering if it is that hard to roll it(4th effect)? Should I continue trying? I ask because in next (I hope) 1-2 weeks I will have another mythic module that is legendary now, and I will need rerolls there too.. I am not salty, just need to know if i am unlucky so I can continue trying. If not and that much rolls need for 4th in general, I can skip it for now, to have some rerolls for the next modules