Every now and then, honest people, using modern technology, spend hours, late in the night, crawling through the forums,looking for a tiny piece of wisdom that would help them do very simple things: synchronize emails, calendars ...
I hate blogging about this, but I have to admit that it is a big relief when you find the magic cure. So, if you're the happy owner of an iPhone 4 and you want to synchronize with multiple Google calendars, this is the info you may desperately look for : https://www.google.com/calendar/iphoneselect once you're logged in.
All other details (why ? how ? where ? etc) are useless.
Good luck !