Inbound calls to one line cause other line to ring
Is the 4 wire cable the type with 2 twisted pairs? If so, you may have the jacks wired incorrectly, such that the each telephone circuit is using 1 wire from twisted pair A, and one wire from twisted pair B. This would lead to increased and perhaps excessive electrical coupling from pair A to pair B.

If the 4-wire cable does not have 2 twisted pairs, then using a 2-twisted pair cable will reduce crosstalk between your two lines (provided you make sure that line 1 uses twisted pair A, and line 2 uses twisted pair B).