Anyone have size suggestions for replacing the stockers? I'm not looking to stuff as much rubber as I can under there, just similar-sized replacements. A little wider would be OK...
My '75 CB750F, which should be the same sizes as your K4 originally had 3.25-19 front and 4.00-18 rear. It is difficult to make direct comparisons dimensionally between "inch" and "metric" sized tires because of different profiles.
My selections were 90/90-19 front, 120/90-18 rear. The front is about as close as you will find dimensionaly to the original, the rear is a little larger. Part of the selection process was evaluating the tire manufacturers specifications on acceptable rim widths. One of my criteria for the front was not to exceed the OD of the original tire (which I could only guess at by measuring the original tire which was still on the bike).
The bike handles nicely and "turns in" very well.
You will find that most people on this forum tend to use the 100/90-19 size for the front tire. There is a wider selection of tires in that size. The 110/90-19 is generally considered to be too wide (it is certainly too wide for the stock rim) and supposedly results in slow handling. I have not tried this myself, so I cannot say from personal experience.
Selections of rear tires vary considerably.