There are some contexts in which εἰς can be rendered "for," but how we translate this Greek preposition may depend on the aspect/Aktionsart of the verbs used in context and the context of a passage itself. Translating Virgil's Latin work Georgics and Homer's Iliad and Odyssey has taught me the importance of examining how a word (preposition or otherwise) is used in context (a term's usus loquendi). Lexicons are helpful in this regard.
What might be helpful is to look at similar texts/contexts and try to determine what εἰς is most likely to mean in a given setting. Certain examples given in BDAG for "εἰς" (2. beta) seem pertinent for understanding 1 Thessalonians 4:15. Compare Matthew 6:34; 1 Timothy 6:19.