Connecting X10 to Arduino
They are lying.

It's not an RJ11 connector.

It is, in fact, an RJ14 connector.

RJ11, RJ14 and RJ25 all use the same shell, but each one has a different number of contacts.

They are also known as 6P2C, 6P4C and 6P6C respectively (6 position, 2 contacts, etc.)

The pins are always centred around the middle of the connector, so an RJ14 connector (6P4C) with the tab downwards and the cable facing you will be:

NC 1 2 3 4 Nc