comparatives - two times or twice

1. In the 1980’s the rate of increase of the minority population of the
   United States was nearly twice as fast as it was in the 1970’s.


2. In the 1980’s the rate of increase of the minority population of the
   United States was nearly twice what it was in the 1970’s.


3. In the 1980’s the rate of increase of the minority population of the
   United States was nearly two times faster than that of the 1970’s.


4. In the 1980’s the rate of increase of the minority population of the
   United States was nearly two times greater than the 1970’s.

Can anyone explain correct grammatical sentence between these ?