A BIT OFF THREAD HERE, but back to Getut's problem:
Getut . . . I think JerryG may have hit part of your answer, small wheels/tires are a often a culprit. And KiotiJohn may be partially correct in how you are using your tractor. He is correct that if you are stuck you really don't want to jam down the HST pedal and expect to get out. On the other hand I think his rules about the use of HI versus LOW range are extreme. I regularily mow and do other chores in HI range, I stay out of low range unless I am digging, somehow engaging the ground, or climbing/decending the hills.
I have several places were there are "dips" in my property for drainage, I've never had a problem crossing them in HI range. KiotiJohn referred to your 4" dip as a little ditch, IF your's are like mine, they are nothing like real ditches. The dips are about 4" deep and about 8" across; just about the right size to get a tire stuck. Mine run across the property in a couple spots, they were put in to drain water over to the creek; heck when the grass gets a bit long you can't even see them, hardly something I'd call a ditch, but hit one while driving a bit too fast and you bounce up in the seat! LOW range is simply too slow for mowing and light duty FEL work, no need for it when using a PHD or hauling a sprayer or a wagon.
My neighbor across the road has a little Kubota
BX1500(?) and he and I were discussing our tractors and he only uses low range when he takes his tractor down to scrape the ditch with a back blade. Now KiotiJohn does make an excellent point about finessing the pedal, but I suspect you may want to have the dealer check out the HST bypass valve setting. The CK20 is a nice small tractor, it would be a shame if it is performing properly and can't get past a tiny dip in your yard. Then again this may be one of those times when my theory of more HP in a small tractor is a good thing, ditto the comments about excess weight beyond what is needed. Your CK20 and my TC24 are very similar in size, your's weighs a little more than mine (about 300#) and has 3 less HP. It might be that the little bit of excess weight and the ~12% lower HP rating are the contributing factors in why the dip in your lawn can't be crossed. But to be honest, I hope that is not what prevents you from getting across the dip in your lawn, I still suspect the HST byplass setting.