To aggregate four switch ports into one L2 logical link with both ends set to active, which protocol must be used?

LACP (Link Aggregation Control Protocol) is the correct choice. It's an IEEE standard (802.3ad) that allows multiple physical links to be bundled into a single logical link, providing increased bandwidth and redundancy. When both ends are set to active, LACP actively negotiates and maintains the EtherChannel. LLDP (Link Layer Discovery Protocol) is a neighbor discovery protocol, not for link aggregation. Cisco vPC (Virtual Port Channel) is a Cisco proprietary technology for multi-chassis EtherChannel, which is beyond a single L2 logical link between two devices. 802.1Q trunks are used for carrying multiple VLANs over a single link, not for aggregating multiple physical links into one logical link.