Question: Among the customers with an account balance lower than 4000, what is the percentage of the customers in the US?
SQL: SELECT CAST(SUM(IIF(T2.n_name = 'United States', 1, 0)) AS REAL) * 100 / COUNT(T1.c_custkey) FROM customer AS T1 INNER JOIN nation AS T2 ON T1.c_nationkey = T2.n_nationkey WHERE T1.c_acctbal < 4000