You know, it does sound like your dog is going into her first heat cycle. First heat can vary greatly from dog to dog. The youngest is generally at about six months of age. Though… sometimes a female will come into season at an even younger age! First heat can start as late as 12 or even 14 months of age or later in rare cases. Again, it can vary dog to dog, just as things vary from human to human. Keep in mind that if this is a heat cycle, she can most definitely get pregnant!
Some common symptoms to look for when your dog is starting to go into heat or is already in heat are:
*Swelling of the vulva. This can vary greatly from dog to dog. Some females swell a lot and others hardly at all. This is usually a good sign.
*Bleeding from the genital area
*Change in personality due to the hormones (backing up, humping, etc.)
At 4 months of age, I highly doubt that the humping is dominance related, but it could be, so you better get it nipped in the bud before it gets out of hand. Interrupt her right away when she is doing it and redirect her to something else, preferably something incompatible with humping. Sometimes humping is an expression of excess energy, just like barking and chewing is. A tired dog won’t hump! Ha ha! Humping is not always dominance, but is certainly a sign of emotion, whether it be anxiety, overexcitement, stress, or even an invitation to play…so a lot depends on the situation itself when she begins her little humping episodes.
I can only hope that you are going to get her spayed at right around 6 months of age!!!!! You will prevent unwanted pregnancies and keep many other serious medical conditions at bay if you do.