Which brand you choose is personal preference choose which features you like the best. Some people like JR, some like Futaba, it's all personal preference.

4 Channels will keep you busy for a while, and is all you need for a trainer. I got a 6 channel for my first radio, so when I upgrade to a plane, say with flaps or landing gear, I will have the channels to do so. You will grow out of a 4 channel radio, you may or may not grow out of a 6, and more than likely will never even use 8 channels (some do, not most).

DSM radios are the latest technology. They allow you to fly without worrying about what frequency you or anyone else is on. FM radios still work fine, and will continue to work fine for at least a while.

I chose a Futaba 6EX for my first radio. 6 channels and computer programmability (which allows you to 'mix' channels together once you get more accomplished at flying. It allows you to store information about each plane, so you don't have to trim your plane every time you fly if you have multiple planes. Mixing allows you to 'mix in' rudder with aileron, or switch between ailerons and flaperons, which again you won't have to worry about for a while.