Design is an ebb and flow type of a career. There are times when you’ll be at a point in a project that work is extremely light. And then there are the times you’ll spend weeks working 60-80 hours a week to try and kill yourself for a deadline.
It sounds like you’re approaching the second part. And trying to leave in the middle of it may not be a smart move. If you don’t land a new job then what?
I’ve been in situations that have been seemingly insurmountable, and how you resolve that is a question of your character. Leaving is an option, but a better option is to become a master of time management, understand what milestones and deadlines you would need to hit to get the work done in 4 months, and figure out ways to jettison work that may not be necessary, or convince management you need additional money to get more man power (consultants or interns) to help the work load.
If you get to the end of 4 months and you’ve gotten the project completed, you would probably be pretty happy with yourself, even if it means a lot of coffee and long nights - because it proved that it was not impossible. More importantly, it will be an impressive portfolio piece with an example of how you were able to take 7 months of work and do it in 4.
Based on what you’ve said about your boss having connections and this being a burnt bridge, it sounds to me like the answer is easy.
Stock up on Red Bull at BJ’s is my advice.