Based upon arriving at 0700 on Day 1 and leaving at 1900 on Day 4 (based upon some random sample dates I've used in October), three (and a half) days parking at Manchester Airport's Long Stay car park costs just £29.99 (which compares very favourably with the £25 I recently paid for a single day's car parking at Stansted). Extending the booked period by 48 hours though does, indeed, make JetPark 3 available, for just £19.99.
http://www.manchesterairport.co.uk/parking/
While the FAQs on the airport's website tell you about the minimum parking time, they don't tell you what would happen if you tried to leave early. The T&Cs don't help much either. They tell you that there's an additional charge of £10 per day if you overstay on your booking but they don't specifically say whether you'd be charged at the higher rate for a different car park (where there's no minimum stay) if to sought to leave early. However they do say this:
"You must enter/exit the car park following the instructions provided to you in your e-mail confirmation and on the car park booking confirmation page. Your booking is only valid for your designated car park on the specified dates. You will be responsible for any additional charges incurred as a result of non-compliance with entry/exit instructions or use of a different car park".
http://www.manchesterairport.co.uk/terms-and-conditions/jetparks-1-2-and-3/
That paragraph suggests to me that they might seek to impose a penalty charge if you tried to leave the car park early (although my understanding of contract law is that no such charge could be enforceable if you'd not been notified about it in advance).
Given that paying the extra tenner saves you having to worry about any problems (and that Manchester Airport's parking charges are, compared to many other airports, remarkably reasonable anyway), I'd simply opt for using the Long Stay car park.