This is just my opinion, but I don't see a price difference between $4 and $8. The biggest challenge is getting me to pay. If it is worth paying for then whether I am going to pay $4, $8 or $10 a month is pretty irrelevant to me. Making me put my hand in my pocket is the hard part. I always think $9.95 sounds like a good price for something on a monthly basis ($100 if you pay upfront for a year also sounds OK).

I'd start high and lower the price (A/B price testing?) as needed.