conversion to judaism - Does someone maternally Jewish, but without documented proof, need "giyur l'chumra"?

If a person is Jewish by maternal decent, but can't prove it conclusively through documents. Do they need to convert? Do they need normal conversion or giyur l'chumra? What are the steps involved? I read about giyur lechumra, but heard that it can only be done back four generations. Is that true?