√(4a²) = 2a?

In proving the quadratic formula I read this

..
(x + b/(2a))² = (b² - 4ac)/(4a²)
x + b/(2a) = (±√(b² - 4ac))/(2a)

Why did they do √(4a²) = 2a? Shouldn't it be 2|a|?
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