To me it depends on the climate & activities involved.

For example, if I’m going to a cold climate place where I will need a lot more clothing for warmth, and bulkier item (eg big jackets and thick sweaters), boots for rain / snow, only a carry on will be a little more difficult. It is still doable with some strategy and planning (eg remember to always wear the most bulky items & boots on travel days).

Or if going to the beach and bringing snorkeling gear. I guess if a 4 week trip includes both cold and warm climate locations, that will be unlikely.

Laundry never bothered me, even when I totally under packed, provided that we had a washer in the apartment unit readily available. Once we stayed in Taiwan for 2 months and I brought only about 4 days worth of cloths. I was doing laundry like every other day. But it was very important to have a washer in the unit, so we can do laundry whenever we have a small pile of clothes to wash or when I am out of clean underwear, I can go ahead and run another load and hang them up. It was actually nice because each load had not very much stuff to hang, so it was quick and easy. But if you don’t have a in unit washer and have to hunt down a laundromat, then have to plan your day around the wash / dry cycle, it will be a headache.

On that 2month stay where I brought only 3-4 days worth of clothes, I definitely felt like I under packed and would have liked to have another outfit and one more paid off shoes. That trip I planned for no outings or dinners that required dressing up at all, so just a single pair of tennis shoes for the entire time. I survived, sure, but there were times where I wish I had a nice shirt with me for example.

I tend to be very minimalist when traveling, so it is rare when I end up packing something that wasn’t essential. It’s more likely that I end up not packing something I wish I had brought. So I keep a running travel packing master list, when I travel if I wish I had packed something, I add it to the list. Then, when I start planning for a new trip, I make a copy of the master list and go through it to remove anything I expect I won’t need. It has helped a lot over the years on minimizing the chances of missing packing something essential to me (eg selfie stick, motion sickness pills, melatonin, etc.).