So I think 4 tenths of a millimetre is a bit sharp.
All real components are subject to a) manufacturing tolerances and b) thermal expansion. Four tenths would definitely be too tight for me (if it goes really badly, you'll slam pressure on a fan -> CPU -> motherboard -> *crack*).

My recommendation:
Look for another case. One that leaves at least 2...3 cm clearance to the fan. There are more than enough enclosures - you just have to look for them (it's annoying, I know; I just put together a complete computer on Galaxus - it's not that much fun, because they completely ignore the most important tech-spec of a fan: "How much power does the part cool down?!" - you have to go parallel to the annoyingly confusing multimedia blinky blinky pages of the manufacturers with all the content-free advertising chatter (tip: just ignore it and always immediately look for and click on the button "Technical data").
I bought the Corsair Obsidian 750D Airflow, for example - plenty of space and a good price (there are fancier ones, but they cost more). It doesn't have the standard RGB blinky blinky stuff, but I don't want an annoying disco, I want a computer.
If you think you can impress others with your great flashing computer instead of naked performance figures, there are always appropriate modding accessories.
Another tip: Depending on what type of user you are, check out the so-called benchtables. For example, I think the Thermaltake Core P1 TG is a good alternative :-)
Thermaltake Core P1 TG (Benchtable)

Yes, that's not a problem. Another user even managed to fit in the Dark Rock Pro 4, which is a few millimetres bigger.

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