For those reading who aren't old timers, this needs to be elaborated on.
This wasn't any trick or sleight on Intel's part. The parts really did run 2x as fast or 4x as fast internally. That was a major achievement. An instruction that took 5 clocks to execute at 33 MHz took 5 clocks to execute at 50 MHz and took 5 clocks to execute at 100 MHz. This was all accomplished over a relatively brief period of time compared to today's rate of CPU speed advancements.
What slowed the CPU down was the growing mismatch between the internal operation and the external memory bus which continued to run at (usually) 33 MHz. It was possible to run the external bus at 50 MHz but most designs didn't. The 8K byte on-chip cache helped mitigate the mismatch.
Actually, the Intel 486DX4 ran at only 3x the speed, despite the name. Intel couldn't use the DX3 name because of a trademark owned by AMD, who also had a part called the Am486DX4. The AMD part was also available with a 40MHz bus and a tripled 120MHz CPU clock.
I seem to recall that right around the time Intel released the 80486 they started getting into the Mhz wars and clock multiplying the hell out of everything. DX2 then DX4. The fastest 68040 maxed out at what...40Mhz, while the 486 ended up somewhere at 150Mhz or so.
But the low volumes definitely hurt Motorola's ability to keep up with Intel's R&D. Clock for clock, the 68k architecture was faster, but Intel figured out how to throw a lot more clocks at the problem and they kept doing that until PowerPCs were not really a consumer-level home computer chip anymore.