Introducing Rescue to Pack

I have (3) dogs and will soon be introducing a 4th - my mom got way in over her head and is asking me to take her pup. My dogs are 13, 4.5, and 4 yo. The 4yo resource guards and can get aggressive if a new dog comes up to me, his toys, food, tries to get on couch or bed. The 4.5yo will follow lead and can mimic that behavior.

How do I go about introducing my mom’s intact 10mos old puppy to my 3 dogs so that he’s accepted into the pack? All dogs are male and mine are neutered.

I’m getting a bit anxious. I’m worried about the dynamic my dogs and I have and whether this will shake it up irreparably.