It's not the adapter or the number of connectors used. It's the connector on the card itself.
I haven't come across anything stating that. Where did you see that information? Everything I read states that approximately 1% of 4090's have this issue and it is attributed to people using 4 pin adapters meaning the card is able to draw up to 600W. I have also read that almost all failures are attributed to the connector not being fully seated.
I have also read a lot of reviews of people with 4090's who are not having any problems. Maybe it's a specific card like the founder's edition?
One other option is to limit the power with software like Afterburner. I have also read that when the 4090 is only connected by 3 plugs, it doesn't draw much more than 430W.
Allowing the card to draw 600W with 4 plug adapters was a bad idea. They never should have done that when the card is rated at 450W. Probably lots of people with PSU's that can't handle that much additional wattage thought it was ok to overclock.
In my opinion, all of this could have been avoided by using the correct gauge wiring. I went to automotive school and have professional training in wiring in regards to gauge, ohm's, amps, volts, etc. One time I was fooling around with an actuator on an electric wheelchair. I hooked up my home made leads from a battery to the actuator. The actuator was frozen. The sleeving on the leads went up like someone lit a dynamite fuse leaving nothing but bare wire. I'm guessing the wiring on most of these adapters is 14 to 16 gauge. That's not much for 450W never mind 600W.