My parents waited on their big female dogs (one Great Dane Mix included) until each had gone through her first heat, then had her spayed. Of course, during that heat, we kept VERY close watch - she was never outside without supervision, as we weren't taking any chances with wandering un-neutered strangers! Almost 4 months seems awfully early, I might call a second vet just to get a second opinion. Why remove her dew claws, by the way? Any particular reason?

And do, if you have one, post a picture - we'd love to see your big baby!