In your CPP-D learning centre, it says that one must be off work for 4 months before one can apply for CPP-D. I am wondering where you derive this number from - I am unable to find where on the Service Canada website that specifically says one must be off work for 4 months before applying. (I am only finding that the disability must be “severe” and “prolonged”).
Can you post a link to where you saw it and a copy of the sentence? The only thing I can find is here https://www.resolutelegal.ca/cpp-disability-retroactive-benefit/
“The only way you wouldn’t qualify for a retroactive payment is if you applied for CPP disability the same month you became disabled, and then Service Canada approved your claim immediately at the end of your minimum 4 month waiting period. This rarely happens unless you have a terminal illness that requires an expedited assessment.”
Maybe it’s because of the 4 month waiting period?