Seems like I too read that, Jimmy said that the F4 was to separate his sound from Bill Monroe's sound. It sure sounds good to me on some of those classic Jimmy Martin recordings.

As to whether an F4 is good for bluegrass, well the above says plenty. Mainly what do you want your bluegrass to sound like. I wish I had one because my band plays a lot of his songs. I borrowed one once, it was a 1921. It had some really great tone. However it did not seem to project where others could hear it as well as my F5L. I always said it was if I was wearing the sound around me. As to chop sounds, they are a combination of player and mandolin. True an F4 is not going to sound like Grisman and Crusher it is going to sound like some Jimmy Martin song.