Well first of all $4000 sounds about right for this kind of case if you get a lawyer. But as far as I know, you don't need a lawyer if your parents (perm residents) apply for GC for you. The only thing is that, if you are already 21, you might have to wait a LONG LONG time to get your GC. Even though the petition (form I-130 that your parents will file for you) might be approved within a few months, you will have to wait a long time to get the visa number [which is quota based] without which you cannot file form I-485, adjustment of status to Perm Res from your current status.
Well friend, the bottom line is that if your lawyer charges you $4000 and can substantially expedite your processing time, then it's worth it.
