The lower the better, in my humble opinion!!!
Like Poppyseed said, as long as the culet is not touching anything, you can have it as low as you want. My OEC moissy and my oval diamond both have bezels that sit that low. In fact I think my oval diamond is even lower than 4mm. However, not sure what the depth is on a 2ct (8mm) OEC, but I think it's deeper than 4.2mm. What kind of mounting is it?!?!
I'm still hot!! Only now, it comes in flashes.