Dear Diane,
Thank you for your message. If you would like to turn off IBC completely then there are two steps that you need to take:
1. In Basic Parameters Under Scope, uncheck the 'IBC' tick box, as you have already done.
2. Under the 'Advanced' drop down menu, click 'Events', and then select 'Clear All'. This second step is necessary because to run the IBC, LEAP runs a series of scripts that checks that your dataset has a compatible structure with the IBC calculation. This includes checking that the base year starts at 2010. Not clearing these scripts is what was causing your error message when running the dataset with a base year of 2004.
I hope this helps. Let me know if you have any other questions.
Best wishes,
Chris