Can I travel with a carry on and a pillow size stuffed animal?
I bought this awesome monkey and I want to know if I could have any problem if I take him as a personal item in a 4 hour plane trip. He weight about 3 pounds and is about the size of a pillow.

Well technically you would be exceeding the carry on baggage allowance of most airlines (which is ONE main carry on bag that must fit into their test unit and ONE smaller item to go under the seat in front of you). It sounds like the monkey isn’t going to fit under the seat. If the flight load is light they may let you get away with it but if it’s full, what you are really doing is taking up TWO baggage spaces in the overhead compartment.
So what you should do is just ask the staff at the check in counters when you get to the airport.