What is the population in millions for 2011 where the GDP (nominal) (billions USD) is 4?
create table table_2155836_1 (population__millions varchar, _2011_ varchar, gdp__nominal___billions_usd_ varchar, PRIMARY KEY (population__millions))
select population__millions, _2011_ from table_2155836_1 where gdp__nominal___billions_usd_ = "4"