You should be able to pick up a power wire from any local audio shop, most of them have spools that they can just cut the length you need. The only advantage to buying kits is getting everything at once. If you lost your power wire, chances are you are also with an inline fuse to protect a new power wire. While I agree that 10 AWG wire is a little small for 500 RMS, 4 AWG might be over kill unless you plan on upgrading in the future. Here is what I would do, go to a shop and ask for about 20 feet of 8 AWG power wire and 3-6 ft of 8AWG ground and a 50-60 amp fuse with holder, if you get a better deal on the kit, get the kit.
To answer your question about what the difference in wire is, wire size is rated in a descending number value as to the diameter of the conducting portion of the wire. 8 gauge is thicker than 10 and 4 is thicker than 8, this will decrease the resistance of the conductor by adding more surface area which will cut down on heat dissipation. keeping the power electrical for the amp and no wasting it on the air...also it keeps you power wire from melting
