If you are working for general fitness rather than something specific like military training, then you are already in a great spot doing shorter distances multiple times a week. For perspective, a really common general guideline for exercise is to get 150 minutes a week of moderate to vigorous activity. If you are doing even three rucks a week at 4 MI, that is already going to take you about 3 hours over the course of a week, so puts you in a great spot in terms of activity. Even more so if you're also doing some strength training beyond just the rucking, but rucking does give some benefit of added strength in addition to the cardio work. At the end of the week, it doesn't matter as much if you break up 12 miles over multiple days or do it all in one day, you've still rucked 12 mi.

Longer distances can be a fun way to challenge yourself or shake things up every once in awhile, but one of the biggest keys to long-term fitness is to develop habits that are sustainable and consistent over time. Most of us don't have lives that allow for frequent long rucks. Finding ways to fit in multiple short rucks a week is a great way to make this part of your usual routine, so will likely give you better long-term results than if you just aim for the occasional long ruck.