I don’t think it counts as a 4 part. Use it as a 3 part. Magnesium will take a LONG time to show a decline, but adding it back in a controlled manner is just to prevent using test kits, which aren’t accurate and cause more headache.Other 3 parts? So this is best to be a 4 part solution?
You can completely omit the extra magnesium and add it as needed based on testing.
If you change water regularly, you may not even need any additional magnesium. Salt mixed usually contain and abundance and consumption is very slow.