Guests gave us 4 stars by mistake instead of 5 stars.

[Asim97]

Guest hit the wrong stars and wanted to give us 5 stars but hit 4 stars.

[Elena87]

@Asim97 

Now, did they contact you first and say 'oh terribly sorry old chap. meant to give you five stars but gave you four by mistake..fat fingers and all that...can I change it? '

Or did you contact the guest and say ' hey why did you give me four stars' whereupon the surprised guest quickly replied 'um it was a mistake'?

If it's option two, it's as likely not a mistake and merely a reflex utterance by a startled guest. 

[Emilia42]

@Asim97 

I agree with @Elena87 but the only way to fix the mistake is for the guest to call Airbnb and ask them to completely remove their review as it is not an accurate reflection of their experience.