Correct Input Sensitivity Setting for Cartridge

Start with lowest possible gain and see if you like it.
4mV is mid, 6mV is high

Just to be clear, we are talking about the output of the cartridge in terms of its signal voltage. We are not talking about the input sensitivity of a cartridge, which is a term that almost makes no sense. Sorry for being pedantic, but if you want to make yourself understood, it is good to get the terminology right.I agree with what everyone else said, try all three settings and go with what you like best. Often the lowest possible gain setting will sound best because of less circuitry in the pathway.

Thanks for the info. As you can see, I haven't much experience with turntables. Formerly I just hooked the table to the Rega phono pre and that was it. Also I thought there may be some considerations about using a modern cart with an ancient receiver. 

Only in that the question arises whether the receiver is in tip-top operating condition.  Otherwise, some of those old classics can be great fun to use/

I settled on the medium setting as that sounded fine to me. Also, the Sherwood is completely restored and sounds amazing. I went vintage for the second system and for under $500 it sounds so good I am wondering why I ever spent so much on the fancy modern components. Some of the old speakers like the AR 5s, Polk Audio Monitor 7s, and Ohm Ls sound excellent, but that is another subject. Thanks all!