If I understand you correctly, you want the data from 4 fields into 1 field.
And from that moment on only use that (5th) field to put comments in for the new records ?
One possible way to go:
Make a comment05_t field - text.
Make a script to put the comment01...comment04 into comment05.
Something along these lines:
Set field (comment05);comment01 & "¶" &
comment02 & "¶" &
comment03 & "¶" &
comment04
You can extend this with more "¶"'s to make more spaces between the comments.
To avoid to adapt all the records manually, put this script into a one time loop for all the records.
To stay at the safe side, just remove after that the 4 comment fields from the layout, but not (yet) from the field definitionlist, you never know.
After a while, when you're sure everything is OK, you can rename them.
I usually rename the fields instead of deleting them....