We have a 4 year old Aussie/Golden Retriever mix. We are about to get a 4 month old Aussie'/ Border

Well, I've fostered only four dogs in the past, but I have good and bad stories about introducing dogs. At once I had four dogs in my hands, two of my own, and two fosters. The new puppy foster was coming home. She bonded great with my other foster who took on role as Momma dog. But one of my own hated her, and still does. To prevent this, you should introduce the old dog on a leash in an open place. Have the other one loose. If your old dog has a tendancy to bark/snap/etc. I'd reccomend a muzzle to prevent injuries to you or the dog.

I AM A FOSTER HOME AND I HAVE NOT YET HAD ANY PROBLEMS INTRODUCING A NEW DOG INTO OUR HOME. I BASICLY JUST BRING THE NEW DOG IN, WE HAVE A FENCED IN YARD AND LET THEM INTRODUCE THEMSELVES. OF COURS I AM RIGHT THERE IF A PROBLEM WERE TO OCCOR, BUT NONE SO FAR. GOOD LUCK

Make sure the 2 dogs meet on neutral ground. A neighbors house is a good place. This way they can get to know each other a little before the newer dog is thrust into your existing dog's territory. The older dog will feel like he is bringing a friend home rather than having an intruder thrust at him.

Congratulations on becoming new parents again.
Keep both on leashes when they first meet, let them sniff and walk around each other. Keep full control of the situtation.
The first week, separate them when you are not at home. Do not cage one, and not the other. Or if you do, make sure the one that is not caged cannot get to the other one.
Play with both, at the same time, praise them both, do the same with each. Never push one away so that you can pet the other.
They must be treated equally, and loved equally.
After a week, you should be able to leave them alone together for short periods of time, until you are sure they are alright with each other.
Feed them at the same time, and the same food. They will check each others food out to see if you gave one something different.
Remember, they are like to children, and should be treated as such.
Soon they will get along wonderfully.
Good Luck!!!!