Run this query:
SELECT CEILING(Total_InnoDB_Bytes*1.6/POWER(1024,3)) RIBPS FROM (SELECT SUM(data_length+index_length) Total_InnoDB_Bytes FROM information_schema.tables WHERE engine='InnoDB') A;
and use the result it gives, i.e., 4 equals 4G. This and lots more great info here:
https://dba.stackexchange.com/questions/27328/how-large-should-be-mysql-innodb-buffer-pool-size