Regarding the quality of the print on your HP4M, it uses a technique called RET for Resolution Enhancement Technology. (I have a HP4 with enough RAM added to do the 600 dpi it can do, and It's great!) Although it does stick to the 600 dpi system with dot positions fixed in that space, it can alter over some range the actual amount of toner laid down at each dot. I'm not sure what range it has. But just suppose that it can do four possible densities of toner at that spot. That is equivalent to dividing the dot position into a 2x2 array and printing a fixed amount of ink in each of 1, 2, 3 or 4 of those positions. In other words, having 600 dpi with four possible toner intensity levels at each dot is the same as having 1200 dpi with fixed toner levels per dot, at least in terms of ability to produce gray-scale differences. It is not quite the same in terms of ability to reproduce very fine detail, but the truth is that most paper and toner systems used for laser printing do not have the surface smoothness to allow accurate dot reproduction at 1200 dpi.
Now, back to your real question. An X10 system sends out a radio signal on the Hot and Neutral lines of the circuit it is on. Now, most households in North America are fed with a 120 / 240 V AC line from an outside transformer. View it as the two Hot lines are the opposite ends of the transformer's secondary winding that produces 240 V AC, and the Neutral line is a center tap on that winding exactly half way between the ends in terms of voltage. Further, at both the transformer and in your house, the Neutral line is securely connected to Earth Ground, so that establishes a reference point. Within your breaker panel, ALL the circuits share the same Neutral line. Approximately half of the branch circuits get their power from one of the Hot lines; the other half are fed from the other Hot line. So the signal sent out by an X10 transmitter already is being fed to half of your house's branch circuits directly. How do they reach the other half? Three possibilities. There may be some radio signal leakage between circuits within the breaker box, although this is probably small. A main route is all the way out through the feed wires to the transformer, through its secondary winding, and back in on the other Hot feed line. This certainly works, but it may suffer from signal strength loss both because the line is long, and because the transformer winding is an inductive coil with a certain amount of impedance (think like resistance) that is worse at higher radio frequencies. So, if those two mechanisms of signal transfer to the other Hot side are not good enough in your house, the X10 suppliers will sell you a third option. (I used to do this with a home-made device for a wireless baby monitor system years ago.) They will sell you a box that plugs into your electric dryer outlet (IF you have one, it is easy to access), and then your dryer just plugs into this box - does not interfere with your dryer in any way. But a dryer has BOTH Hot leads coming to it so it can use the full 240 V AC for its heater coil. The add-on box contains a suitable capacitor connected to the two Hot lines, and it allows high-frequency radio signals to pass through from one to the other, without letting any significant low-frequency (power line is at 60 Hz) power to leak across. So this device establishes within your house (shorter path) an efficient route for radio signals to reach the other half of your house wiring. That way ALL of your house wiring becomes a signal transmission system for an X10 transmitter's signal, no matter where it is.
Of course, this means that all your signals COULD get out of your house and over to your neighbor's if they also are using X10 and connected to the same transformer. And the other way around. So X10 systems have a way for each householder to set their own unique house code, plus use a set of 256 different device codes that apply within that house code. In that way you and your neighbors all can use these systems without interfering with each other.
