But if 3+1 is enough, then why would asus for example put 16+2 into p6t(oldie)? Cleaner signal?
Heat, Stability and capacitors life. For capacitors, more warm they get, less life they get, they can be rated for 85ºC or 105ºC. It's usually go in that direction : Mosfet (Where the heatsink usually go) -> Inductor -> Capacitor. So the heat from the mosfet can radiate to the capacitors, especially on badly cooled design. Current design have poor but beautiful heatsink (On B350) but are still overkill for normal use, even with 1.45V, like Woomack said. You can still reach throttling temperature with a good overclock on most if not any B350 boards with 4+2 Design if correct conditions are meet (Watercooling so you don't have the usual airflow from the CPU heatsink, and burning tools like mprime). The only board I was able to reach high throttling temperature in normal operation (without my specific cpu melting tools), was the Gigabyte Gaming 3. They also fixed there heatsink issue properly with there new X470 board (replaced beautiful heatsink design with fins because heatsink should have as most fins as you can pack there).
But yeah, a 4+2 design is way enough for Ryzen provided it's cooled properly so I would not get a board that don't even have any heatsink there (some MSI don't if I remember). Also, for the Gigabyte Gaming 3, you can find a .STL file so you can 3D print yourself a air duct to link the back fan to the VRM heatsink to fix the problem, If someone need it I can find the link or attach the .stl file.
Edit : For stability, I don't know if it really affect anything like people say. My Asus B350-Prime plus get warm out there but it does seam to change anything. I would also add that more warm are the VRM, more less efficient they get and in
theories you can get a runaway effect once you reach a specific temperature+current consumption. VRM temperature related CPU throttle are set to prevent that to append.
Edit2 : having more mosfet will split the load more so they will get more efficient and should produce less heat too (in the same time too much VRM mosfet can neut the efficiency bonus).