In the context of Indian Railways, "WL 4" stands for Waitlist number 4. Whether you can travel depends strictly on the type of ticket you hold. If it is an i-ticket (purchased at a physical counter), you are technically allowed to board the train and travel in the unreserved compartment or seek help from the TTE to find an empty seat, though you do not have a guaranteed berth. However, if it is an e-ticket (booked online via IRCTC) and it remains "Fully Waitlisted" after the final chart preparation (usually 4 hours before departure), the ticket is automatically cancelled and the money is refunded. In this case, you are not permitted to board the train, and traveling on an e-ticket that failed to confirm is considered "traveling without a ticket," which can result in heavy fines and being detrained at the next station.