When you exposed properly for Zone VII and tried to print it to be Zone VII, you found the Zone IV was too thin, printed too dark.
I think you underdeveloped. You have a flat negative. You may have chosen the time for D23 stock instead of for D23 at 1:1.
Recommend re-doing the test, keep it same - the tan towel at Zone VII wasn't a bad idea and the black t-shirt falling around Zone IV should give you a good range to work with.
But develop for 13 minutes or more.
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