Intel i486 DX4/100
The first 100MHz ticking part Intel brought onto the x86 market. The DX4/100 was actually a DX3/100 as it didn't clock quadruple but clock triple the front side bus frequency.
In many cases an old 486 couldn't be upgraded to the DX4/100. Not only because some motherboards didn't understand the clock doubling or tripling technique, also because the voltage requirements changed. Older 486 processors use 5V on the core instead of 3V for this DX4/100. Of course it did boot up at 5V but it can damage the chip. Not to mention the excessive heat output it generates at 5V!

