If an unsecured debt is over 4 years old, is it true that you can no longer be sued for it?

As a general  rule, no, this not true, though it may be true in individual states for specific causes of action.

Every obligation to pay a debt arises out of some legal cause of action. Usually, in the case of unsecured debts, it has arisen out of 1) a written agreement to pay or promissory note; 2) an oral agreement to pay; or 3) an "open account"--like a revolving charge account. (Note: some states don't distinguish open accounts from written contracts or agreements.)

Each cause of action has what's called a statute of limitations, or time limit on how long the creditor has to bring a legal action (sue) to enforce the debt. Once the statutory period has passed, the debt is unenforceable.

Each state sets its own statutes of limitations. It may be the case, that for the cause of action underlying your particular debt, the statutory period in your state was less than 4 years, in which case, yes, it would be late to enforce it. But to know for sure, you have to check the statute of limitations for that cause of action in your state. If you repost your question with more detail, someone here should be able to look this up for you.