to answer the question of the original poster...
A 4.5 oz shirt is in the "designer" or "fitted" shirt category. These are fine if someone wants a very light shirt that is more fitted than a standard men's box cut shirt. Usually the 4.3oz shirts are also made from Ringspun Cotton which gives them a much softer feel as well.
As for Regular box Cut shirts go with the 6.1oz shirts like the gildan 2000, Delta 65000, hanes beefy t, etc. The shirts that are made in this category that are in the 5.2-5.6oz weight don't hold up as well as their 6.1oz brothers and I only pick up the 5.5oz shirts when someone is doing shirts as "promos" or if they really just want something that is really light(for the summer).
So Regular fit shirts Go with 6.1oz.
For fitted shirts you can definitely go down to 4.5oz. hanes offers them in that weight. Delta offers a 4.3oz men's fitted shirt(style 11600). Delta even offers a 3.3oz junior ringspun cotton shirt for ladies. It's in junior sizes, but I usually end up picking up L,XL, and 2XL for ladies and it has a longer length too. Those look really comfortable.
Hope I helped a little bit.