There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status (fulltime), a relationship test and residence test. The support test is slightly different for each type.
So, if you're over 18 and not a full time student, the $4200 income limit governs whether you can be a dependent.
If your income was less than $4200, the support test, for a qualifying relative, is that your parents provided more than half your support. as opposed to the support test for a qualifying child, that you paid less than half your expenses.