Hi By now hopefully you will have found the other posts about puppy biting (or mouthing) & realise that you have a little 'cockerdile' to deal with. But, as others say, you also have to help your 4 year old understand that the puppy is a sentient being & needs to be treated with respect.
Cockers are very sensitive dogs and respond best to positive training - much like 4 year olds
If it becomes a battle of you constantly having to tell him off all your lives will become a misery.
Perhaps you could enlist his help in training the pup? When I have pups I keep handy a 'cleaning up kit' of bowl, super-absorbent cloths, single-use gloves, poo-bags, kitchen towel, anti-bac wipes & plant sprayer with vinegar/water solution. Your son could be in charge of this. 4 year olds tend to love dressing up - you could give him a badge or a hat & give him a title such as 'Accident Patrol'. A 'star chart' would be something he can relate to - I'm sure they have star charts at pre-school. You could introduce a star chart for your puppy so that your son can give them a treat & a star everytime the pup 'goes' where they should. In the event of accidents your son will understand that the routine is to sigh, put on the gloves & clean up. You can discuss with him what went wrong - perhaps the puppy scratched on the door when playing - you could ask him if he thought she was asking to go out & suggest that next time they play he lets her out if she does it again? He can watch to see if she's getting bitey - what does this mean? Would she like a chew? Is she sleepy now? Does she want a wee?
This way you can teach him to 'listen' to his puppy, to see her as a creature who needs care & understanding.
You will be teaching your son some valuable lessons for the future.