That’s way longer. 20cm is the average blade length for chef’s knives. Over here in Germany, with way more liberal knife laws: Legal to own (duh) but also very much not legal to carry much less wield in public unless you have a good reason – like actually preparing food in public. You can transport that kind of thing without fanfare but transporting very much involves not having it at the ready.
Four months are still completely overkill, though. Impounding and maybe a week’s worth of fine (one day of disposable income == one day in prison here) if he was being stupid and careless but non-aggressive. Four months go way beyond “let this be a lesson” territory and very much into “the state is nuts and doesn’t make sense”. If you’re feeling poetic, how about some social hours in a charity store sorting donated fidget spinners.