Sadly, Intel dropped plans to introduce a 4GHz Pentium 4 in favour of the (not compatible) Dual Core/Core Duo technology. Dual Core is essentially a cheaper, stripped down Core Duo (with 1Mb L2 cache instead of 2Mb). Core 2 Duo performs better.

You may be able to overclock your processor, but it's not for the faint-hearted - reliability could be compromised, and there would be a lot of heat to dissipate. The results wouldn't exactly be startling, either!