I don't think he is too young to be taught table manners at all. However, I think that the behaviour you describe is pretty 'normal' at this age.
Rules I try to enforce with my 2 eldest dss (4.5 and 2) are:
Eating with mouth closed - ds1 does this, ds2 still struggles a bit (he's 2)
Sitting on bottom on chair while eating (this is a tough one to enforce - my two are so wriggly and impatient and just want to mess about - I'm constantly telling them to sit on the chair instead of kneeling, stop wriggling, sit properly - we are getting there - ds1 is quite good now)
Waiting at the table until everyone has finished (again, they don't do this naturally and quite often just get up but I call them back straight away)
Asking to leave the table (ds1 does this, ds2 doesn't yet but I'm starting to reinforce that with him)
Taking their own dishes to the sink (both do this)
Help to put mats and cutlery out (both do this)
I don't have any issues with using fingers to eat - neither of mine do this. I was at a friends the other week though and they had bolognese pasta and her ds (4.5) was eating with his spoon in one hand but shovelling more in with fingers of his other hand (made feel quite sick actually!). She didn't pull him on that but I'd be horrified if my ds did that - particularly in company!
I'm not saying all 4 yr olds will have perfect adult manners but certainly they are not too young to be taught even if you do have to keep reinforcing the rules at every meal.