To find a relative date 18 months in the future from 4th May, you can manually calculate it by first adding 18 months to the 4th May. Since a month has varying days, you can break it down to ensure accuracy. For instance, adding 12 months first would take you to 4th May the following year, and then adding another 6 months to get the final date. By doing so, you can accurately determine the date 18 months in the future from 4th May.