Brilli's proof of 0 = 1

Brilli the smartest kid in a school, gives a proof to the Brilliant community stating that 0 = 1. At what step number does it become unequal?

Here is his proof:

Step 1: $-20 = -20$

Step 2: $16 - 36 = 25 - 45$

Step 3: $4^2 - 4 \times 9 = 5^2 - 5 \times 9$

Step 4: $4^2 - 4 \times 9 + \frac{81}{4}= 5^2 - 5 \times 9 + \frac{81}{4}$ {Just added it}

Step 5: $4^2 - 2 \times 4 \times \frac{9}{2} + (\frac{9}{2})^2 = 5^2 - 2 \times 5\times \frac{9}{2} + (\frac{9}{2})^2$

Step 6: $(4 - \frac{9}{2})(4 - \frac{9}{2}) = (5 - \frac{9}{2})(5 - \frac{9}{2})$

Step 7: $(4 - \frac{9}{2})^{2} = (5 - \frac{9}{2})^{2}$

Step 8: $4 - \frac{9}{2} = 5 - \frac{9}{2}$

Step 9: $4 = 5$

Step 10: $\large \color{#20A900} \therefore 0 = 1$