the networks will remain hybrid for some time
for those who can't get 3G thanks to poor coverage, phone uses frequencies for 2.5G or EDGE, blackberry does that alot, once a blackberry user complained that his blackberry was damaged because it didn't had edge, i explained him that edge is what you have when you don't have 3g or 2.5g

4G won't cover entirely the country so the slower networks will remain in place for a long time
most gsm phones have 4 bands now, 850, 900, 1800
some has 1900 and recently 2100 depending on what network uses it for 3g and 4g/lte
4g is for data only for the moment, so phones will use gsm frequencies for calls and 4g for internet