wow I would love to have a 4~5 carat diamond ring! I also think the cartier catalog is a hint as well. But...do you know if she tried that type of ring or a yellow diamond ring? I fell in love with yellow diamonds when Tiffany's came out with their yellow diamonds line but when I tried them on a few times in different lightings, I realized yellow diamonds don't quite work on my medium yellow toned skin. Sometimes, the bright yellow made my hands look "dirty" 
I much prefer off-white stones on my skin so my "darkness" don't get emphasized. However, I won't turn it down if I was proposed with one in the size you are looking for! But that's mostly because 1. I value size&cut more than colour(w~z range is cheaper than colorless range), 2. I sometimes like yellow diamonds on me, and 3.chances of me getting a diamond in that size is slim 
I love jewelry and tell my bf bits of what I like but I doubt he really knows what I want for an engagement ring because rings I love to look at/admire and what rings work for me(suit me) are different.
I love surprises too but I don't want to be left out on choosing a ring I will have to cherish and wear forever. I also don't think knowing the shape of the stone or setting will make the proposal any less exciting. If I can't choose the ring myself, I would really like to give my bf a range of designs and let him choose from those.
But then again, that's just me. My sister likes to dress up but doesn't really care about diamonds. She let her bf choose and she is happy with what she received.
If you really really wanted to completely surprise her, I suggest choosing a ring from a vendor that offers a wide range of diamond shapes and full refund. She might do some more researching on her own without telling and her taste might change and want a particular brand of diamond… like August Vintage Cushion?
I much prefer off-white stones on my skin so my "darkness" don't get emphasized. However, I won't turn it down if I was proposed with one in the size you are looking for! But that's mostly because 1. I value size&cut more than colour(w~z range is cheaper than colorless range), 2. I sometimes like yellow diamonds on me, and 3.chances of me getting a diamond in that size is slim I love jewelry and tell my bf bits of what I like but I doubt he really knows what I want for an engagement ring because rings I love to look at/admire and what rings work for me(suit me) are different.
I love surprises too but I don't want to be left out on choosing a ring I will have to cherish and wear forever. I also don't think knowing the shape of the stone or setting will make the proposal any less exciting. If I can't choose the ring myself, I would really like to give my bf a range of designs and let him choose from those.
But then again, that's just me. My sister likes to dress up but doesn't really care about diamonds. She let her bf choose and she is happy with what she received.
If you really really wanted to completely surprise her, I suggest choosing a ring from a vendor that offers a wide range of diamond shapes and full refund. She might do some more researching on her own without telling and her taste might change and want a particular brand of diamond… like August Vintage Cushion?
good luck!
