On version 3.4.X:

H:> zabbix_get -s 127.0.0.1 -k system.run["echo On a 3.4.X Agent a right square bracket does work ]"]
echo On a 3.4.X agent a right square bracket does work ]

On version 4.0.X:

H:\>zabbix_get -s 127.0.0.1 -k system.run["echo On a 4.0.X agent a right square bracket does NOT work ]"]
ZBX_NOTSUPPORTED: Invalid item key format.

I have tried all sorts of quotation and escaping to no avail as I get the error 'ZBX_NOTSUPPORTED: Invalid item key format.'