Downloading may or may not increase supply of images, it isn't clear.
I think the most hard core paedophiles most likely trade images and videos among themselves, and they feed off each other.
Some of these images will make their way, free of charge, onto the wider internet, and I would guess at this level an image might be viewed thousands of people. The people on this level should not be viewing/downloading these images, but they are not on the same level as those who actually groom and molest children.
Also some cases may be borderline, e.g., 'nudist' images, which might feature young girls are I believe legal, but might be treated as such by the police. There are plenty of blurred lines of course, e.g., if you looked for legal images and found illegal images, then the illegal images would remain on your computer unless forensically wiped.
There are other things like images/videos where the performer is assumed to be 18+ but is later found to be underage. Pornography of an 18 year old with a petite figure, wearing a schoolgirl uniform, etc., is perfectly legal, whereas pornography of another person who looks older but is actually 17, would be illegal.
I'm not clear what the four counts relate to with respect to Rolf Harris, whether they are four images or more, and what kind of images/ages they are, so I couldn't comment further on that, it could mean anything from a large collection of young children being abused to some borderline images downloaded by mistake.