"NL2 Speakon cables (2-pole) are compatible with both NL2 and NL4 panel jacks, while NL4 cables (4-pole) only fit NL4 jacks, creating a one-way compatibility. A 2-pole plug works in a 4-pole socket because it uses the 1+ and 1- contacts, but a 4-pole plug is physically too large for a 2-pole socket."