how many 4ga wires can you get off of 0ga, etc.?

Just use whatever size is necessary for the equipment you are powering.  The rule of thumb is to simply use the proper gage wire for the total load.  4 AWG is good for up to 150 amps.  1/0 is good for up to 350 amps.  Always use a larger size then the maximum load you expect to carry.  So for example, if you are running three amplifiers, each requiring 100 amps, I'd run one 1/0 from the battery to a distru blaock, and then use three 4 AWG, one for each amp.  And the proper fuses, of course.

I dont see a real problem with the way DYohn stated, just use ohms law rms/idle voltage then find out what size wire your needing cause if you are pulling 150amp 10-13 ft I would use 0awg and then split it with 4awg there should be no problem at all.....