sweetheart wrote:

I have just bought my husband a Karcher K4. Now OH tells me it cannot
be used on a mains supply because it is too powerful. Is this true?
it does say "cannot be used on the water net"). So, can it be used
or not?
What it actually says is "the appliance must never be used on the
drinking water net without a system separator."

Which sounds like Karcher-speak for "should have a double check valve on
your garden tap" but you should have one anyway.