Those numbers refer to the last 4 digits of whatever phone number is posted in the ad, and are just meant to be a search tag. The CL search function will only locate text within an ad that is identical to the search term you've entered; if a phone number is written with spaces or characters between the digits (for example), the search function won't return any matches unless you've specified a search term that exactly matches what is in the ad. You'll find that a number of CL posters deliberately list their phone numbers in a way that's not easily searchable.
In lieu of a phone number, try a search term/keyword like a name or particular service offered (e.g. 'greek', 'bbbj'). Hope this helps.
