That makes sense, except there are other modifiers besides +0, +2, and +4. If you look at the Conventional infantry Range Modifier table on pg. 149 of the TechManual, you will see that there are mods of +0, +1, +2, +3, +4, +5, and +6. So assuming that anything beyond +4 is long, you still need to decide what to do with +1 and +3. Basically do they "round up" to Medium and Long respectively, or "down" to Short and Medium, respectively.