Rockwell and WDC had 65C02's up to 4 MHz relatively early on, but the 4 MHz versions seemed to be quite rare. WDC now has 65C02's rated at 14 MHz, but they go quite a bit higher than that if you've got fast enough RAM.
There are some technical details on why a 4MHz Z80 is roughly equivalent to a 1MHz 6502. As always with processor design there are tradeoffs in every decision. The Z80 had a 4-bit ALU, but I'm not sure if that slows it down.
The Z80 has a more complex architecture than the 6502. A 6502 clock cycle is one bus cycle and simple instructions can execute in one clock cycle. For the Z80 a clock cycle is called a T-state, and one machine cycle consists of multiple T-states. A simple instruction like INI takes 4 T-states.
There is also some tiny pipelining at play: the 6502 needs to access (read or write) memory on every clock cycle. A 2-cycle instruction reads the opcode on the first and the actual work is done on the second. This leaves the second cycle's memory access open for doing something cool like fetching the next instruction's opcode.
There are some technical details on why a 4MHz Z80 is roughly equivalent to a 1MHz 6502. As always with processor design there are tradeoffs in every decision. The Z80 had a 4-bit ALU, but I'm not sure if that slows it down.
The Z80 has a more complex architecture than the 6502. A 6502 clock cycle is one bus cycle and simple instructions can execute in one clock cycle. For the Z80 a clock cycle is called a T-state, and one machine cycle consists of multiple T-states. A simple instruction like INI takes 4 T-states.