Knowing nothing about UUID v4 generation, I have likely a stupid question. What makes you so confident that all implementations and their entropy sources are flawless enough to make actual collision probability close enough to theory?
In the end we have to make some assumptions about the correctness of (some of the) components.
So depending on the consequences you might opt to reduce the risk or help mitigate the consequences.
To just state that it is about as likely as my coffee maker being a portal to the future isn't very helpful. Poor entropy sources or bugs are not uncommon.