Well done for having a go and getting them all shorn. It could be that no 4 was just not ready to shear. The way to check is go over the sheep before you start, seeing if you can see a rise - the pantaloons are usually last and can be ready to shear as much as 2 weeks later than the rest of that sheep. All the sheep are unlikely to have a rise at the same time. For example, we shear our tups at least 2 weeks before we start on the ewes, but their shearing can be spread over a couple of weeks, meaning some tups will be shorn a month before some ewes. First shears are often ready much later than old hands, and sometimes the rise is impossible to detect in a first shear at all.
Try pulling the wool in a small 'pinch' away from the skin, while holding the skin flat with your other hand. You may have to wiggle the top hand a bit to get the wool to separate - it's really just roo'ing. If the new and old growth won't separate, then check in a week, and again in another week. The glory of blade shearing is you don't have to do all your sheep on the same day, which is a boon when you have lots.
If you found the actual clipping of the wool on no 4 was very much more difficult than
the rest, leaving your hand sore, then almost certainly that sheep wasn't ready and would have benefited from another week or so. Shearing should be very easy with the blades gliding through the fleece when you have found the rise, and your shears are sharp. You might need to sharpen them (another skill to learn) before you use them again.
The first time I saw a blade-shorn sheep I burst our laughing because compared to a machine clipped animal it had loads of fleece still. How much wool is left depends on how far above the skin the rise had reached. If you are left with only 1/8th of an inch of new wool, then you could have left your sheep a while longer. We usually have perhaps half an inch of wool left, which reduces any temperature shock for the animal if the weather is very hot or cold, which can be a problem with machine clipped animals shaved bare. Don't forget that wool insulates against both hot and cold.
If you have shorn no 4 a bit early you may find that it sheds the old wool at the tips later in the summer, or that it's still there next year - remember which sheep it was to explain any oddity at your next shearing.
Again, well done
btw - I hate football