Lecture notes illustration on p.28

Hello, Why in this example (Illustration-Revaluation & annual excess depreciation transfer) the historic depreciation is counted as 5 years? 1 January 2020 which was acquisition date to revaluation date which was 1 January 2024 is 4 years when I count it. Am I missing something? This gives me different result to new depreciation cost as I count remaining useful life as 21 whereas it's 20 years remaining in the example. Could you clarify? Thank you