The older bias-ply tires of 40+ years ago were supposed to be 100% profile, a 4.00 tire was 4" wide and 4" from bead to outer edge. So, fitted on an 18" rim, you'd add 4" + 4" to that to get a 26" diameter. But, there were small variations that wouldn't show up until after mounting, giving slightly different diameters. On your 4.00-18 vs 4.00-19 question, both are supposed to be the same width. Now comes the goofy part. When P-metric's appeared, there was some confusion over size equivalencies. A 4.00-18 was supposed to be the same as 100/100-18, but P-metrics were mostly available as 90% profile, and a 100/90-18 would be a smaller diameter, by almost an inch. So, a 110/90-18 was a better sized replacement (90% of 110 gives 99mm, almost 4" bead to outer edge). Over time, as P-metrics started taking over, we found some supposedly old-style tires were actually re-labled P-metrics, I guess to maintain market share or save money. Started to distrust tire sizings after that, and begun to rely more on actual mountings and customer reports. That said, the 4.50-18 may actually be a 120/90-18. A 4.50-18 should be (18" + 4.50" + 4.50") = 27" in diameter. The 120/90-18 should be (18" + (90% of 120) + (90% of 120)) = 26.5" in diameter...
