Eliminate 3-way switches, one at a time
First you go to one, single, 3-way switch, and you eliminate that switch.
As you may know, 3-ways operate by connecting the common pole to either one of 2 travelers. So in any particular switch position, one traveler is connected to common, and the other is orphaned. You're just gonna make that permanent.
Pick one 3-way switch. Turn the power off. Decide which traveler you want to use (i.e. which color) and unscrew it. Also unscrew the common. Splice them together in the normal way with a wire-nut.
Put another wire-nut on top of the abandoned wire, so that it can't short against anything. If the wire nut doesn't stay tight, go down a size (e.g. yellow to orange). Put some electrical tape over it to make sure it stays. Don't nip off the wire. Leave the physical wires in place just in case you do something in the future.
At this point, turn the power back on, and test to make sure everything works. That is to say, all the surviving 3- and 4- way switches should operate normally, i.e. the way they worked before. If they don't, go back and revisit what you did.
Now, do the same with the other 3-way switch. Remember which travelers you chose to use, and which ones you chose to abandon.
Now replace the 4-way switch with a plain switch.
The 4-way switch is a funny one. It has 2 travelers coming in, and 2 travelers coming out. Remember you abandoned 2 travelers. Turn the power off, remove and cap off those two abandoned wires.
The switch should still work -- with only 2 wires on it. It effectively works like a plain switch at this point.
This is an important step. If you leap straight to the smart-control and stuff doesn't work, you'll have no idea why.
Now that it's all working as you want, you can think about eliminating a redundant wire run. For instance if power is flowing from one box to another then coming right back on a different wire uselessly, you could get rid of that redundancy. Again, one step at a time, and test in between.
Now install the smart gadget
Now you follow the smart gadget's instructions for replacing a single switch.