Frankly, I think your friends are being grossly inconsiderate.
I would never dream of including my husband in a "girls' outing" just like that, and certainly not when the one person is newly widowed. I am truly sorry for your loss and for the inconsideration shown by your friends.
I understand that you don't want to hurt them, and why you don't really want to go. In your place, I think I would ring up and say that you think it would be better if the two couples went on their own.
If asked why, simply say, that although you are happy for them that they still have their husbands, you are not yet ready to be the widow included kindly in two married couples' outing.
That what you had been looking forward to was seeing your two women friends on their own, so it wouldn't cause too much fuss if you broke down and cried.