Depends on what you mean by "the beginning". Its all a perspective thing after all. For me, the first book is the beginning for sure.

The beginning for Roland would be impossible to pinpoint imo.

The beginning for Roland's story is whatever Gan decides it is at this point. We learned in the very first book that Roland was destined for the Tower but there was so much more we didn't know. Learned more in W&G and those who consider the comics cannon are learning even more.

But who's to say its even the same loop?

And can we even say for sure that Roland's memories of Mejis and Susan in book 4 were even part of the same loop we were in?

One assumes his memories of Mejis are set regardless of the loop--the other assumes it could all be changed.