I had the 4D's, comfy but leaked easily. Partner has had 3 pair of the x ultra and loves them. No leaks for him but none of his boots have leaked where as all of mine do.
The 4D is a bit stiffer, like an hd hiker. If you want light and nimble the x ultra is probably the better choice.
I have a pair of the Quest 4Ds that I use for my Fall - Spring boots. They’re fairly comfy with just under a year of daily wear on them and they’re still water proof even with the rand separating. I had to wash them in the creek last week when I stepped in dog poop and misstepped, foot stayed dry. I’ve used them for walks around the neighborhood, down the new coastal trail in my area and to the butte about an hour from my place (3 mile loop with 1600ft elevation gain/loss). Tread life still appears to be great and with a set of thick wool socks my feet haven’t gotten cold yet (single digits is the lowest I’ve stayed outside for prolonged periods with them). They also have good traction on packed snow and ice.
I wear the X ultra Wides. I buy them when I can get them for around $100. I get 1-2 years out of them. I wore the regular X ultra’s prior to the wides, just half a size bigger.
They fit me better than any other boots, no blisters with good socks. I’m still wearing them right now or until we get some moisture.
They aren’t the most durable or waterproof. Not much ankle support either. It seems if water comes up to the laces they leak. I’m good with the lack of durability for the weight, price and comfort.
I have both, my x-ultras 3 are the leather edition (highly recommended upgrade) my x-ultra 2’s were more comfortable, 3 sole are stiffer but not quite a boot but not really a shoe. Comfortable but the interior of the shoe seems to be deteriorating much faster the my series 2
Any quest boot is really good, they fit me great, and I can(and have) taken them out of the box and went on a 10–15 mile hike and not an ounce of discomfort only boot I’ve not needed to break in