What is the difference between the Canon 70-300mm 4-5.6 USM lens and the Canon 70-300mm 4-5.6 USM "L" lens?

They have very similar specs, so why the large gap in price? is there a significant difference in terms of quality and performance? Why is the L version almost 4 times more expensive than the non-L version? Is the price gap justifiable?
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