Hi Ljr
As to it being 4GB, that isn't really a problem in itself and if the cause was a long running transaction, which it looks like with the undo files being present then you will want to find the source of that transaction to avoid a repeat. FYI i had a DB server with a 90GB ibdata1 before I moved to file_per_table and cleaned it up.
Run show explain for xxx being the processid for all current processes to see if you can find what might have a stalled transaction holding the undo.
Have a look through the above page for additional help to identify the culprit and if it is still there only dormant waiting for some action
Cheers
Peter
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