feline agression

Question
I have four cats, two of them are 4, one is 2 and we got a kitten 5 months ago. I have 3 males and one female. The four year old male is giving me trouble. All of them get along with the kitten except for the 21 pound 4 year old male. He started getting nasty with the baby. He hissed and slapped him a few times. Then he started atacking the other cats. This cat used to be the sweetest cat, would not hurt a fly. Resently we had a few stray cats try to get in our yard. I don't know if he is angry with the strays coming into our yard or what. He gets really mad when we won't let him out and he turns on the cats. What should I do?

Answer
I would suspect the strays are aggravating him.  Try to keep the blinds/curtains closed.  Put a harness on him and let him have 15-30 minutes outside to roll in the grass and explore.  Also, I would recommend adding several FELIWAY Diffusers in the house as well as RESCUE REMEDY to the water dishes to help reduce stress and anxiety in the house.