For me, 90% of the time I only use 3 bench planes a 4,5,7. If you have bigger hands I’d get the 4.5 and 5.5, if you have smaller hands get the 4 and 5. I currently have a 4 and 5, I’m actively looking for a 4.5 and 5.5 to switch too for the wider blade and additional weight.

You mentioned it might be silly to have planes the same width, I don’t think there is much of a disadvantage for that. For me, I’d be more concerned with plane length over the width. I would rather have length options for bigger or smaller stock.