Male or female dogs, to add to present family?

We have 2 male castrated shi tzus they get on well most of the time, they are 3yrs ad 2 yrs old,we love this type of dog and have a large house to home them, we do not want to breed them, we are thinking of getting 2 more, baring in mind the of the present dogs would it be best to get 2 males or 2 females to complete our family, what ever I get they would be neutered. My only concern with female is that a few years ago we had a female west highland terrier and she used to have phantom pregancies and get very moody and snap if you went near her bed, she had not been neutered. would 4 boys get on ok.?