H.W.
She may be a perfectly nice person to you, but let me explain: if her perception is that "he spent the hours yelling and screaming. basicllly being difficult" then that's probably how she's experiencing it. (this is neither a right or wrong) I nannied for years, and I only ever suggested a parent take their child to the doctor when there was evidence of medical need. ( hives, ear infections, even once, a periscoping colon, which is a medical emergency). The point is, babies cry. A lot. If a person is providing care for a baby, they need to have a temperament to endure a lot of anger, frustration and tears and still have a relatively positive perception of the child in their care.
And even on frustrating days, parents need to hear what worked, what went right. I've been on both sides of this.
A question: has she ever asked what works for you when he cries, or has she told you "I tried X and then Y, but it seemed like it wasn't working for him"? Also--are there other children whose needs require her to leave him on his own a lot? (Supervised, but without adult interaction) I just ask this because, if this is her job to watch your son for 4 hours and he's crying "constantly", it sounds as though his needs (social/emotional) aren't being met. Research shows too, that holding and soothing are very real needs for infants and toddlers. Children are certainly adaptable, but infants need responsive adults who are in touch with their needs and personality--it is not the burden of the child to fit in with the preferences of the caregiver in this particular regard.
If it were me, and I had a child who was crying for longer than 40 minutes after the parent left, I would have called the parent to check in. The fact that he's crying for 4 hours is worrisome.
Find someone who loves babies and who really doesn't mind holding or wearing your son. Your sitter may have had a very different relationship with her son at this age too, and your son's needs may be out of the range of her personal experience. I don't want to slight your sitter--she probably has the best intentions and has drawn her knowledge from her experiences. And she might be terrific for kids who are more interested in being alone, ready to explore and play more independently. But children of all ages need to feel secure in having their needs met before even beginning to engage in exploratory or constructive play. This may be what's happening--I don't know.
I'll be interested in seeing others posts on this. My apologies for this being so long. My background leads me to believe that caregiver/child relationships are often more nuanced and complex than most people perceive them. It sounds like you are a loving, concerned mom who is striving to find balance in your son's life. It's okay to stretch our little ones from time to time, but this sounds like less of a stretch and more of a gap. I hope you find someone who thinks your son is just the terrific little person you think he is!:)