I m not in favor of used tyres & even while buying new tyres one shud check its manufacturing year. But if u got some good tyre @ very low price then u shud consider used one. BTW presnce of tread on tyre is not the only health sign for a good tyre. With passage of time rubber bcomes hard even if not used. some time unused tyres or old one also got de shped which causes bubbling etc type problem as well.
I m bit cofused regarding size of tyre u mentioned. coz 4.00-18 tyre wud be 4 Incehs wider with 80% hieght of its width & same goes for 3.50-18 & 4.50-18 as they wud be 3.5 & 4.5 inches wider respectively.
I mean above mentioned sizes are diffrent so how u r saying that previous 4-00 18 tyre was of 4.50" wider & recently bought 4.00-18 is 3.50 Inch wider? Basicaly all these sizes are diffrent from other & have their own width.