What can I do?

i got a lettle from a lawyer the other day saying that i owed a dr some money that my ex told me he was going to take care of 4 years ago. that was the first i knew anything about owing the money i recived no bill on it till the letter from the lawyer. should i let them know that my ex was to pay it 4 year ago. i get ssi and very little child support and won't be able to pay it what am i to do?
if i would have got the bill i would of don't something about it before it went this far.

You can certainly advise them that the bill is the responsibility of your 'ex' and not yours, but that may not stop them from their efforts to collect from you (depending on other factors... community state issues, actual agreement, etc).

However, their attempts to collect will very probably be hindered (or stopped altogether) if you could show them that any actions against you would be a waste of their time and money. This can be done by showing that you are 'judgment proof', without seizable assets or likelihood of having any. After all, a creditor is not going to waste money going after a debt that HONESTLY has little or no chance of getting paid. They may very well 'ding' your credit file, but will probably not spend on additional pursuit.