Communication between device for 5 meters and low speeds
The TIA-485 standard does not specify what kind of cable you have to use for RS-485 networks. Twisted pair is very frequently used because it's cheap and works very well for most scenarios.

For your particular application I would say it depends on how noisy your power supply and loads are but at such a low speed unless you have to deal with a horribly noisy environment you should be good.

I would like at it from a different angle though: you're wasting quite a lot of copper there. But I guess 5 meters is not that long, but you might consider replacing your four-wire with a two-wire for power plus a twisted pair that would be probably cheaper and lighter.

If you are asking for a theoretical development of this problem a very good reference is this one: Noise Reduction Techniques in Electronic Systems by H.W. Ott. In there you will find the how and why of the twisted pair.

Another good reference to look at is: Serial Port Complete by J. Axelson. This book will give you a lot of tips on the practical side of RS485.