This is too hard. I plan on doing something similar except instead of work 4 months every year and take the rest of the year off, I plan on working about 7 more years and never working a day again.
http://www.mrmoneymustache.com/2013/02/22/getting-rich-from-...
The basic idea is that if you can save a fraction f of your income for the next n years, then you can live work free for the next n*f/(1-f) years after that.
You do this by changing your lifestyle such that f is big and you enjoy your life. Then when you stop getting paid, you can happily carry on like that. Obviously you should increase your income as well!
7 years of work at 90% savings = 63 years worth of cash.
Once you're saved a reasonable chunk of money (~$500k) it starts to look after itself (if you invest sensibly) and you can make it last indefinitely.