4-Way light system controled by just one dimmer
For standard switches, that is true.  Only one of the switches can be a dimmer (it needs to be a 3-way type) and the other switches are conventional on/off 3- way and 4-way switches. Each of the non-dimming switches turn the lights on at the level set by the dimmer switch. But to change level, you need to go to the dimmer.

There are more sophisticated systems that use a master and slaves. In these systems, each switch can turn on and off the lights at a preset level, or they can override that setting and each control brightness. These switches need to be a matched set, require neutral wires as well as hot, and cost a good bit more than conventional.