That currently sounds perfectly sufficient. Check out the limitations of the Express version so you know if/when you'll reach a point where you'll need to seek out a more encompassing version.

DISCLAIMER : Not a Codeigniter Person, Just a MySQL DBA

Looking at the flow of control in the PHP. It says to me:

• Check the following mod_id,course_id,user_id,interaction_id
• If not found, INSERT the new row
• If found, UPDATE all the other columns

There are three(3) mechanisms provided for INSERT and UPDATE scenarios

MECHANISM #1 : REPLACE INTO

REPLACE INTO `scorm_interactions` SET
    `mod_id` =  '4',
    `course_id` =  '5',
    `user_id` =  '185',
    `interaction_id` =  'Question2_1',
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
;

or

REPLACE INTO `scorm_interactions`
(`mod_id`,`course_id`,`user_id`,`interaction_id`,
`interaction_type`,`time`,`weighting`,`correct_response`,
`learner_response`,`result`,`latency`,`objectives`,`description`) VALUES
('4','5','185','Question2_1','choice','10:45:31','1','Knees*',
'Knees*','correct','0000:00:02.11','Question2_1','');

This mechanically performs DELETE and INSERT on mod_id, course_id , user_id, interaction_id

MECHANISM #2 : INSERT ... ON DUPLICATE KEY

INSERT INTO `scorm_interactions`
(`mod_id`,`course_id`,`user_id`,`interaction_id`,
`interaction_type`,`time`,`weighting`,`correct_response`,
`learner_response`,`result`,`latency`,`objectives`,`description`) VALUES
('4','5','185','Question2_1',
'choice','10:45:31','1','Knees*',
'Knees*','correct','0000:00:02.11','Question2_1','')
ON DUPLICATE KEY UPDATE
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
;

MECHANISM #3 : SELECT ... FOR UPDATE

Did you know you can perform an exclusive lock on rows you intend to update with precision?

According the MySQL Documentation, you can run a SELECT query on the very row and issue a row lock along the way. For example:

SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;

This will lock the entire table and allow an UPDATE of the counter_field column.

In your case, simply run the two queries back-to-back

SELECT * FROM `scorm_interactions` 
WHERE `mod_id` =  '4',
AND `course_id` =  '5',
AND `user_id` =  '185',
AND `interaction_id` =  'Question2_1'
FOR UPDATE ;
UPDATE `scorm_interactions` SET
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
WHERE `mod_id` =  '4'
AND `course_id` =  '5',
AND `user_id` =  '185',
AND `interaction_id` =  'Question2_1'
;

EPILOGUE

As a safety cushion, try increasing innodb_lock_wait_timeout if the traffic is write-heavy. See my StackOverflow post : How to debug Lock wait timeout exceeded?

Give it a Try !!!