Why 4 motors?
Most tanks I see use one motor per tread.
I suppose that if they are mechanically linked, 2 motors could be used for a single tread.
I don't want my tank's speed so slow
This is an arbitrary requirement. What is slow?
On #1, if 12v is the right voltage, get any 12v battery. After that, the number of AmpHours just correlate to run time. If your tests indicate that the battery runs low too quickly, add more of the same battery in parallel.