Two guys are out - they have to leave a tip - one says to the other "how much should we leave?" - other guy almost almost always says "I dunno - 5 bucks?" I feel like 4 mph is a little bit the equivalent of 5 bucks. It's almost always the answer to the question of what pace should I try and maintain while rucking.
Personally I think rucks are best for aerobic conditioning at a fairly conservative heart rate (aerobic threshold/zone1 or 2, MAF rate if you're a Maffer). Lots written about the benefits of that type of training. In hilly or even hilly-ish terrain you will have to slow down on uphills to stay below target rate . You may be able to step it out on the flats and hit 4 mph but if your average pace will still be < than that depending oh hills.
So I'd adjust weight depending on what you want to achieve and importantly, the terrain. If you can't hold < max HR rate on climbs, even at a meaningfully reduced pace, you probably have too much if you goal is aerobic training. In "Training for the uphill athlete" House /Joshnston/Jornet talk about capacity training v utilization training. Capacity obviously builds long term potential performance, utilization builds near term performance results. In my opinion the average person should think of rucking in terms of the former - unless you're prepping for an event or course. I rucked a fair amount in the military (so I get the 4 mph thing...) and I can tell you that long rucks with heavy loads will definitely take it out of you - which to me doesn't make sense for the average person.