You had too much draw on your board. Do you really have 100cm(3 feet) long risers?

If they are actually that long, your GPUs would pull too hard and that's why you had a burnout.

The only way you could hope to make that work on more than 3 cards would be to undervolt your cards before putting them on risers. If you have them on the stock voltage and software, the card will pull whatever it needs(remember that the power on will momentarily pull max power through every port for POST). If you're on a 100cm riser, you're pulling WAY more than a 1x slot can put out when it's already probably struggling to feed the cards on the 16 pin connectors.

It's a hard lesson but smart people only have to learn it once.

You'll probably have to replace that board but when you do, stick with 3 cards max unless you get shorter powered risers.