For highland, USC and UCSD all they did was incorporate the intern year (probably because fewer people were willing to apply for prelims when so many other programs didn't require it). They didn't really change from 3 to 4. Just from 4 to 4 all in one place.
Harbor switching over is the only true 3 to 4 year switch.
So I disagree that there is a "push" to go to 4 years. A lot of California were already 4 years and have been for ages. California has a huge population of uninsured patients: Medicare doesn't pay for uninsured patients. So California programs being 4 years has more to do with having a bunch of true county programs that don't really depend on Medicare funding than anything else - so when the government stopped paying for the fourth year a bunch of California programs didn't HAVE to switch over to 3 years.
85% of the rest of the country is 3 years...us Californians are just weird