Table schema:
CREATE TABLE table_2155836_1 (population__millions VARCHAR, _2011_ VARCHAR, gdp__nominal___billions_usd_ VARCHAR)

Question: 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"