[dTal]

I am curious what form of energy you think the other 4% turns into?

Both systems are 100% efficient at turning electricity into heat. Neither system is 100% efficient at turning electricity into useful hot water. There are always losses. They're just being honest with their accounting.

[SigmundA]

The electronic components that create microwaves (magnetron, transformer etc.) are only 50-70% efficient at making microwaves the rest being lost to waste heat. In order for this device to be 96% efficient they must be capturing waste heat in the water through some sort of heat exchanger.

Whats the point then? A resistive element immersed in water converts nearly 100% (probably some light losses at the elements base) of the energy to heat in the water its immersed in.

So on one hand you have complicated expensive microwave system that still resistively heats the water in a much more complicated way losing 4% still to waste heat or you have a much more simple resistive heating system that probably does better.