I currnetly have a four channel JR S400. It looks like a fifth channel could be added easily to the radio but I have no idea. Can it be done or do I just need to get a new radio?

I'm guessing that your basing this on the fact that there are additional holes behind the battery for servo reversing switches. This is because they use the same molds for multiple radio models.

If you want more channels, you will need another radio.

actually I had the radio apart awhile ago to make some repairs and it looked like there was space on the board for another channel, but you are probably right, infact I figured that but was just wondering if I could add flaps to my MUS w/o spending big $$

When we use flaps, we usually find it is necessary to mix in a little down elevator. This can be done by hand, but the most effective is to use a computer radio with mixing. There are other advantages to having a computer radio. You might want to wait until you decide to get one.