The pentium 4 is an OLD single core chip. Alot of versions were made, on sockets 423, 478, and 775.
Some run at high clock speeds above 3 ghz, but all have long pipelines, meaning the number of clock cycles it takes to actually do something is higher. So you dont have very good single-task power.
Some have hyper-threading technology, which means for every physical core it has one virtual core. So its a virtual dual-core. But thats still not gonna give you much multi-tasking power.
The i7 is a very modern physical quad core chip with hyper-threading, high clock speeds, turbo-mode (even higher clock speeds up to 3.7ghz), and very efficient architecture. It is a single and multi-tasking beast. However for most people the i5 will be enough, as the i5 is basically an i7 minus hyper-threading and most programs cant use 8 cores anyway. (i7 has four physical and four virtual cores)
That enough?