The OP should really learn to search the forums or google around. In the time it took to make this thread, Google would have had the answer in seconds. At any rate, there's a multi-threading topic in the Development forum (a long, good disucussion, not the closed one on the first page) that details a lot of why Dolphin is at most going to use 3 major threads.
IIRC, Dolphin had experimentally been tested to use more cores, but there wasn't a boost like one would expect. If you split a program's tasks into smaller parts to run simultaneously, you need to make sure all of those parts are kept in sync, which can cause a lot of programming headaches, and defeat the performance gain you'd like to see. It's not always a matter of throwing more cores at the problem. Also, I think the GC/Wii hardware itself is ill suited for quad-core emulation. The way both systems were designed isn't easily broken down so that you could emulate it with 4 cores. Of course, I'm not a dev, nor an expert on multithreading; this is just info I've picked up here and there.
IIRC, Dolphin had experimentally been tested to use more cores, but there wasn't a boost like one would expect. If you split a program's tasks into smaller parts to run simultaneously, you need to make sure all of those parts are kept in sync, which can cause a lot of programming headaches, and defeat the performance gain you'd like to see. It's not always a matter of throwing more cores at the problem. Also, I think the GC/Wii hardware itself is ill suited for quad-core emulation. The way both systems were designed isn't easily broken down so that you could emulate it with 4 cores. Of course, I'm not a dev, nor an expert on multithreading; this is just info I've picked up here and there.