Currently:
SELECT cu_name,ck_kt_no
FROM cu_customer, ck_customerkeys
WHERE cu_no = ck_cu_no
AND cu_subsid = ck_cu_subsid
AND ck_key = 'examplecode';
Please do
SHOW CREATE TABLE cu_customer;
SHOW CREATE TABLE ck_customerkeys;
And use aliases in the SELECT. -- I can't tell which field is in which table.
That would tell me to which table does ck_key belongs.
And you may as well show us the 4-table query.
Also SHOW TABLE STATUS -- want to see table sizes.
(Sorry, I prefer output of the above statements over trying to express the equivalent in English -- too much change of an important item sloughed over in the translation to English.)
innodb_buffer_pool_size should be set to about 70% of _available_ RAM when you get to your 16GB machine.