These things are very individual, and the thickness of the ring is important as well, but 4.5 will be far too large for most decent Air Rifle shooters, and many of them will have the tube extended out as far as permitted under the rules. 4.2 is usually the normal place to start, but by all means go up a wee but - but many will be shooting < 4.0. For smallbore standing, quite likely smaller again as the aiming mark is proportionally smaller than the 10m target.