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Unless you are trying to modify/revive an existing addon in which case you may want to avoid an extensive re-write,
it's best if you use the frame manipulation API both to create and script your frames from lua, without using xml.
As to your actual question.
#4 is deprecated and should not be used (it means Blizzard can disable it at their convenience since they've warned it's to be phased out)
#3 and #2 are equivalent, the difference is only in style as long as the key to the table is a contiguous string.
If you had spaces in the name then table["two words"] needs to be used t.two words will not work.
#1 makes an implicit lookup in the global table but is otherwise the same as #2 and #3.
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