Are parts 2, 3 and 4 ignored or are they canon?

I believe they are ignored. They're going with the Halloween playbook.

The Illuminati shut down the Sawyer/Slaughter murder programme back in the early 1990s, which caused a lone Leatherface to move to the orphanage.

No one knows, this series is the worst as far as rebooting its own continuity

To be fair, it's not like the 80s/90s sequels followed the original with much effort or continuity.