"The sprinkler company says that it takes between 10 and 25 CFM to properly blow out the sprinkler lines. I have a 4 gallon/125psi compressor but the specs don't include it's CFM rating. Anyone know what it is for this compressor or how to calculate it? "
That is a basic compressor spec and should be on the compressor, in the manual, or at the manufacturer's website. Failing that, take a look online at similar compressors and you will get an idea of the possible CFM range. A compressor the size you' re talking about is not going to be anywhere near 10-25 CFM.
I blow my system out with an old Sears one that puts out 6.6 CFM at 40 PSI, with maybe a 10 gallon tank. It's not ideal though. I go through the zones twice. Each time I let the compressor build up to about 75 PSI, then turn on the zone. The compressor can't maintain that pressure, so it starts dropping. But it does a good enough job.