What amp do you have that is "four Ohm" and what speakers do you have that are "eight Ohm"? Do you have other speakers on this four Ohm amplifier or is it dedicated to the patio speakers?
8 ohm speakers will not hurt a 4 ohm amplifier. There just may be a bit more of a damping effect than you may want. Some people can't even tell anyway!
probably as far as I know there won't be damage on amp(rather on speakers), however I don't advice you to put together if you wanna make all night rock, rave,etc.. party. If you understand me what I have in mind... But result may not bee good for your taste.