1- Asking for an exemption from the Canadian authoroties to allow me start counting the 730 days after i finish my 4 years in the UK.
2- Simply do the landing and stay for 4 years in the UK, then going back to Canada, and when the customs officer asks for how long I had been out of Canada, I would answer 2 weeks for example !
3- Giving the PR up and reapplying after 2 years for example. In that case will my previous declining of the PR affect the future application, kind of being not serious ?
1- Not really an option, as Leon said, when you apply for PR, it's because you intend to settle in Canada, and not just to have a card under your sleeve;
2- If you spend 4 years in Canada, you'll necessarily lose your PR, because you cannot fulfill your residency requirements, even if you spend all of the 5th year in Canada. Sure, you can try lying, but there's a chance you'll get caught.
3- You do not need "give up" PR. You can simply land and go back to the UK. Who knows, life changes, maybe you'll find out the job is not so good, or you could get fired, or you could decide you had enough... Then, yes, after 3 years, you will not be able to meet the residency requirements, it makes sense to give up (not quite sure it is possible, but I think so) and re-apply.
As to your last question: I don't know if they care whether you had PR and lost it. But the new rules state that to get PR (at least through the Skilled Worker program, which is what I assume you applied for), you need to have experience in one of 38 selected jobs. Which means, if you give up your PR now and re-apply, you may actually not be elegible.