CREATE TABLE table_2155836_1 (population__millions VARCHAR(255), _2011_ VARCHAR(255), gdp__nominal___billions_usd_ VARCHAR(255))
What is the population in millions for 2011 where the GDP (nominal) (billions USD) is 4? 
    
SQL: SELECT population__millions, _2011_ FROM table_2155836_1 WHERE gdp__nominal___billions_usd_ = "4"