It's kinder not to ask, midori1999, imo. To ask and hear the response and then go ahead anyway is jerking the child around. The inevitable, unnecesasry and completely avoidable problems that arose from the OP's approach prompted her to post.
I disagree that the way to ease a child's fears is to throw them in the deep end, so to speak. The dog's presence in the house after he repeatedly told his mum he was afraid and didn't want one is quite "in your face", imo.
I see nothing wrong with a child being afraid anyway, and my priority would be to keep lines of communication open, let the child understand his feelings are ok and that he can share them with me and expect them to be dealt with respectfully (not 'tough, we're getting one anyway'). I really don't get why the process had to be accomplished on everyone's timetable and everyone's terms except the child's. Why the rush? I have observed that children who are confident of a parent's understanding and patient ear develop far more self confidence and resilience, and are kinder to other children too, than those who have been forced to wrap their minds around things according to the dictates of a parent who has pooh poohed their feelings and fears.
I am not advocating allowing the lunatics should run the asylum 4 year olds to make decisions about too much that goes on in the home. I am opposed to forcing children to come to terms with a fait accompli after paying lip service to the notion that he could have some say inthe matter. The equivocation does not hold water.
I question the motives of the OP in getting the dog "My partner wanted a dog for his birthday and has always wanted one and we felt my son was now old enough to cope, and we decided to get a puppy so it would be totally unthreatening. We reasoned that once the unknown element was gone he would warm to it and think it was fun." She herself clearly outlines the decision-making process, beginning with 'My partner wanted a dog for his birthday..' She later started enumerating some of the benefits of having a dog, but I don't think these were her primary concerns when she was making the decision.
She also prides herself on not mollycoddling the child -- as far as I can see, there's definitely no danger of that. There's lot of middle ground between mollycoddling a child and letting him understand his pov is of no consequence.
The OP seems to have set herself up in opposition to the child (he won't want to go to school but he will go and it'll be good for him, etc.) and seems to expect that the milestones of his life will be characterised by some sort of battle of wills between the DS and her no need for this either. The OP would make life easier for herself and far less traumatic for her son if she tried meeting him in the middle and understanding and truly trying to accommodate his perfectly valid pov and feelings.
Just because he's 4 years old doesn't mean he shouldn't have fears, opinions, or feelings that don't coincide with his mother's. He has a right to inconveniently require sensitivity and shielding from things that scare him to the point of climbing up furniture and squealing in fear.