mpeg-2 aac vs. mpeg-4 aac

hi, all,

I am a little confused here. What is the difference between MPEG-2 AAC
and MPEG-4 AAC? They all have main profile, LC. Can I see them just as
different profiles? Is there any difference in the bit stream format?
Do they require different decoders?

Also the licensing, does MPEG-4 AAC also cover MPEG-2 AAC?

Thanks
syl