Leading edge Recovery

I have been getting calls from leadng edge recovery when i called them back they needed my last four digits off my social security number which i did not give they then were rude to me and told me that i had a outstanding debt from 2003 from a credit card that isnt showing up on my credit report. Can they potentially add this to my report, and should i send them a debt validation letter. Or should i just let it go they have the right address but the name they have is mispelled and is a partial of my last name. Its only for 400 dollars and a friend of mine said in new york that they really wont come after a debt that is under 500. Any advise or help would be greatly appreciated