There is nothing inherently better about 4 GHz. Using GHz as a metric of processor performance has been highly misleading since about 2000, and was primarily perpetuated by Intel. Around that time AMD and Intel designs started to diverge, with AMD opting for doing more work in a clock cycle, while Intel continued pushing for more cycles that do less work.
To paraphrase from AMD's consumer level documentation, imagine an adult and a small child walking together. They both cover the same amount of distance, they both go the same speed. However, the adult (AMD) takes significantly fewer steps than the child (Intel). The child has to take many short, fast steps to do the same work that the adult does in one slow step.
This analogy is somewhat dated now, as it is comparing the AMD K8 Architecture vs the Intel Pentium 4 Netburst Architecture.
It is my belief that while 4GHz will eventually become commercially viable, there will be substantial diminishing returns for the cost benefit one gets from faster processors.
Heat and power have already been touched on in this article. Another issue is propagation delay. With each new generation of processors, the transistor count grows. The progression is approximately linear, with two cores having approximately twice as many transistors as a single core (technically there are more due to the interconnection overhead). So for a given core, as new instruction sets are added, better branch prediction is implemented, et cetera, the number of transistors between the data entry point and the data exit point grows. Each transistor takes a minute amount of time to respond to a change in voltage. Although this time is practically nonexistent, when you put a few million of these in a row, that time starts to add up. The longer the chain, the greater the minimum time a clock cycle can be.
This is why we are seeing the sudden expansion into multiple cores and multi-threaded programming. We are very close to the limit of how fast we can realistically make a processor. So now, to get more performance out of them, we are increasing the number of cores they have. Eventually you can expect to see special purpose and general purpose multicore hybrids (like the Cell Processor on the PS3). GPUs will also follow suit.
This is strictly my opinion, mind you, and applies only to the current architecture of AMD/Intel processors. Additionally, this is all null and void if we move to quantum computing or some other new foundation.
