The 4400 is actually wider around than the 3400, as well as taller. From the HMG website the 3400 bottom circumference is 33.5 inches, 40 inches top circumference and 34 inches tall, while the 4400 has a bottom circumference of 38 inches, top circumference of 45 inches and is 40 inches tall. i.e (33.5,40,34) vs. (38,45,40).
I was recently in this same position, trying to decide between the 3400 and 4400, and I ended up ordering the 4400. I've only had it for a few weeks now, and no overnight trips, so I can't give it a serious review, but so far it seems like a great pack.
My logic with going with the 4400 was that it is less than an ounce heavier than the 3400, but a substantial increase in volume, but with the side cinch straps, you can really make the 4400 appear and act like a much smaller pack. I wanted the extra volume, even if I didn't end up using it: it is easy and cheap to get smaller and lighter packs, but buying another larger volume pack for week+ cold weather packrafting trips would be expensive.
I know in the various 'pack fitting threads' people would have you believe that having any weight less than a millimeter away from your center of gravity is going to cause a torque so severe that your torso will rip in half, but the wider circumference of the 4400 is a big selling point to me, even if it might move the center of gravity a bit further away from my back. That's because I like packs that are shorter and squat rather than tall, as I find the very tall packs very difficult to bushwhack with. I'd rather the pack be essentially shielded by my back as much as possible, or else it tends to get caught and just gets in the way when navigating thick bush. Even though the 4400 is "taller" than the 3400, the large circumference means you can get more volume filled at a lower height.
The lack of load lifters of the 3400/4400 is also a selling point for me: I don't like it when tall packs come too close to your neck and shoulders because it makes it difficult to crane ones neck to look up steep hills during scrambling/bushwhacks.
I also find that PFDs and packrafts take up a bit more room in the pack than their exact volume. Sleeping bags and clothes can squish to fill available space. A folded packraft is essentially undeformable, so it ends up creating some "useless" dead space unless you can find something to fit around it. I much prefer having the packraft inside the pack rather than strapped to the outside.
I also find foam PFDs to be incredibly bulky items, and they seem to take huge amounts of volume. Paddles I like to have strapped to the outside, so they don't really eat into the packs volume.
Other than that, I am quite happy with the HMG 4400 so far. I have seam sealed it, and it is very nearly water tight, even when fully submerged. I've gone on a few day trips with about 25lbs in the pack, and it carries like a dream, very comfortable.