Very long transmission-line design tips
RS-485 is good for 100kbps at 1.2km, using two wires for data and two for power.  You could probably stretch it to 2km at 1200bps. 

Here is a TI application note for a 2-wire power + data solution.  It couples the data transceivers through large capacitors, and connects the power source and sinks through normal-mode transformer filters.

1000BASE-LX gigabit Ethernet can run 5km over fiber.

A reflection may take 40us to travel 4km at 10^5 km/s.  At 1200bps, you have 400us settling time from the leading edge of a bit to a sampling point a half-bit away.

In the comments, the OP mentions using untwisted, unshielded pairs.  This will hurt noise immunity.  Consider using parity bits, CRC's, retry protocols, or error correction.