Xbox one turns off after 4 seconds

Well indeed the second scenario that you suggested is what happens.

What do you mean by Overflow?

That's a good question, a reflow is where you heat the southbridge chip to melt the solder ball array under the chip to reconnect any broken connections between the motherboard and the chip. It's quite a difficult and tedious process. Here is a tutorial for the Xbox 360, but it is the same process for the Xbox one https://youtu.be/B4PPAfOvhTk. You don't need a big machine like his, just a heat gun and a way to control air flow. Also kapton or some type of heat type to shield off other components is a must need. Also use solder flux to keep everything cool. Now you don't need to remove the chip, you can first just try to apply flux around the chip and heat the chip until you starts to see the solder balls melting. You can see under the chip if you look at eye level with the board. Count to 15 after you start seeing the solder balls melting, then cut the heat. After that, let the board cool for about a half hour, then see if the console will boot.