1. In AAC-MPEG4 block size is 1024 or 2048?
2. It is not clear relationship between 49 bands and 24 critical bands. How to divide spectrum in to 49 bands by following 24 critical bands?
3. Can you please tell me the final encoded AAC (MPEG4) frame structure with all necessary bits?
4. Is it enough to have only MDCT, Pshychoacoustic and quantizer for AAC encoder by neglecting other components like TNS, PNS, LTP,etc.?
49 scale factor bands account for not only ERB's (not critical bands, please) as well as signal behavior for cases where there are time delays and decorrelation between channels.
MPEG-2 AAC works on 1024 samples, which means it's got 2048 samples in the window. For MPEG-4 I am not sure. I'm not a fan of the sound of super-low bit rates.
Describing the frame structure? Go buy the standard. That's what standards do.
For anything below medium rates, you want TNS.