I have a UK tourist visa, which expires May 12, 2018. My nationality is Kuwaiti. I plan to visit the UK on May 7, 2018 and leaving one day prior to my visa expiry. I need your advice in this regard.
You are allowed to enter the UK any time up to the day your visa expires. You must leave before the visa expires. Within those limits your visit is allowed.
You will probably find that you are asked some detailed questions about why you are visiting the UK for only four days, because that is an unusual thing to be doing. You will need to be able to answer them.