I need to make a communication between only two devices with a distance of 5 meters, which are connected by a cable of 4x2.5mm² (see in the image), being 1 wire for GND and wire for 12v, that would pass nominal current lower than 10A, and the other two wires will be for communication. I thought to use rs485, would it be possible to use 4 way cable without being twisted for a 5 meter distance and slow speed rate (1 byte every 50ms at most)? Would you have any other suggestions? Thanks in advance.
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.