By four they should be backed and well behaved on the ground. I should be able to hack them and ride them with the basic go stop aids, all paces and some basic shoulder in, leg yield and turn on the forehand. Basically should go forward off two legs and sideways if I put on one leg. I don’t worry about an outline unless they offer it that will come later and they don’t get worked more than 4 times a week. My four year olds have all gone to shows even if they haven the competed.
I'd expect it to behave while handling it, stand in cross ties, behave at the farrier, load on lorry/trailer and can be lunged in walk trot and canter. A 4 year old can be saddlebroken already, I'd want it to be able to walk trot and canter normally, nothing forced with abnormal high knees or flexed necks.
I wouldn't have any training expectations (in the saddle at least, handling is important though) at all to be honest. Backed, walk trot canter halt turn would be ideal if I was looking to buy; if not backed then I'd be fussy about their tempermant, ground manners and free movement. It would be nice if a 4yo was going at prelim level a but as a lot of warmbloods develop later they only get backed towards the end of their 4th year when they're physically and mentally ready. What you're buying when you get a youngster is their breeding, movement and temperament essentially, not their schooling but it is always nicer if they have the basic basics in place if you want to be able to ride them soon after purchase. Depending on your own experience and abilities, some schooling and maybe training show experiemce makes life easier but it is not a deciding factor (for myself at least) in young horses.