As already said, a P4 is a desktop chip (used also in some cheaper laptops). There was a P4M, too, which was a mobile version (I still have one). They have better battery life, but not like the Pentium M in the Centrino laptops (and newer ones). After the P4M they reduced clock speed and increased cache memory (which supposedly made the chips do about as much) and battery life got a lot better. Regarding a used laptop--I would be careful if you plan to use it on battery power. A lithium ion battery only lasts so long, and if you're buying one already 2 or 3 years old it might be almost dead. If you are going to run it off a plug, then no problem.
Someone mentioned ripping a DVD to your hard-drive. That must be a Macbook Air fan, as that computer has not optical drive. Ripping to your hard drive with a commercial DVD is not easy because they are encrypted to prevent this.