insanity well I guess you have justified OP's anxiety! Glad to hear your children are doing well now.
OP, I've been in the situation where I knew something was wrong but everyone said I was worrying about nothing, and it turned out I was right, so I'm not trying to get you to write off your instincts. I do want to give you more information to help you make more sense of what is bothering you about your baby.
The short version of what I was going to write about scratching is:
Nearly all babies scratch so some of them will go on to be diagnosed with autism. Most won't.
All babies go through silent phases. Some will go on to be diagnosed with autism, most won't.
All babies have periods of being overstimulated and tired, and will turn their face away and avoid eye contact, ignorIng people. Some babies are more easily overstimulated than others, so their natural levels of turning away and avoiding eye contact will vary (personality factors I suppose you could call it). Some babies will go on to be diagnosed with autism.
As one of the specialists a GP or HV would refer on to, I will repeat it is extremely unlikely that a 4 month old baby who avoids eye contact and likes scratching will go on to be diagnosed with autism rather than it being a typical developmental stage. This is because all babies between the ages of 3 and 6 months have periods of avoiding eye contact and doing lots of scratching etc.
As insanity points out, her children who have been diagnosed with autism did some of these things you describe about your baby. I'm not sure from Insanity's post whether they were doing these things at 4 months old or if they were older.
I'm not dismissing your worry about what you saw on the you tube video.... I want to give you food for thought in the other direction. If hand scratching by babies was a reliable marker of autism, wouldn't it be a widely known thing for people to look out for? And screened for? The checklists of things at each post natal and infant check would include it. It would be on the diagnostic criteria for autism. It would be something routinely asked about (in the way that a social smile is asked about at the 6-8week check). im not writing this to argue with you or to make you think you are wrong, just to give you some points to ponder on the other side of the scale to weigh up in your mind, if you see what I mean
ive got a doctor's appointment now but I'll come back and finish off a few thoughts later. I hope you manage to have a lovely afternoon with your friend.