Do you mean in GP4 tweaker? If you have enabled real time edit for those, GP4 tweaker should save the changes upon exiting tweaker, not GP4.
Close GP4, re open tweaker and you'll see new values for those positions you've modified.
Then when you re open GP4 all your changes should be present.
When you are happy, disable the real time editing so you don't mess the positions up by pressing the wrong key in game..
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Tahitian GP Circuit
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Easter Island Circuit
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