The Ontario Highways program has the next 4-laning part start in "2024-2025" between Highway 529 and the newly completed portion of twinning north of Highway 522. That's about a 20km stretch, so a fairly significant chunk.
Another project to extend the 400 Northwards by 12km towards Shawanaga is also shown as starting in "2024-2025".
This will leave a ~35km gap.
Very good news, I hope they can get it all done soon. I recently drove to Sault ste. Marie the other day and I'm surprised that a large chunk of 17 between it and Sudbury is still two lanes.