You can buy chain breakers for around £25 but you may prefer to do it the hard way if you are not doing this that often. 04B is perhaps the more difficult chain to break because the pins are so small. Drill a small hole in some aluminium plate (6mm thick upwards) of a diameter larger than the pin diameter. Place the pin of the chain where you need the break over the hole in the plate. Using a large parallel pin punch and hammer, drive the pin downwards until the pin is flush with the top plate of the chain. Now with a pin punch that has a diameter smaller than the chain pin, continue drove the pin through the chain. You may need to put some spacers under the aluminium plate if the pin hits the bench below.
For chain breakers, see our friends at https://chainbreakeruk.co.uk/
Hope that helps