Fenton ... I have purchased a few of the K4 decal sets and applied a few. I bought samples to study their accuracy to prototypes for which I have studied/collected data (specifically DT&I and Ann Arbor RR). A lot of what I found is quite accurate with regard to sizes and specific lettering (data, reweighs, etc.) but not everything. For example, on the Ann Arbor caboose set the compass herald is oversize making it unusable for the cabooses. But the compass size makes it correct for using on the DT&I boxcars that were assigned to the AA (in a timeframe beyond the scope of this list) and partially repainted with AA heralds and reporting marks.
Not knowing the thought process or planning that went into the production of certain sets, I can't help but ask "what were you thinking?" Another example: I bought one of the Ann Arbor FA-2/RS-1 sets to see if the numbers could be used to renumber the Rapido AA FA-2 models (size, color, etc.). They look suitable for this task BUT there is no "4" in the number jumble (the AA units were 50-50A through 56-56A). Again ... "what were you thinking?"
On the positive side, the K4 decals are offered in an amazing range of scales (from G to Z). So it is possible to get obtain things like heralds in different sizes needed for other freight cars. I'm attaching a photo of a DT&I flatcar that needed a tiny compass herald with the "Connections" slogan. The N-scale K4 set for DT&I gondolas with coil hoods provided the correct size for the HO model shown. Need a large herald for the side of a water tank or shop building? Look at the O-scale or G-scale version of the K4 decal.
Application: the K4 decals are well printed and have nice thin film. I ran into no issues applying them like any other decal.
Bottom line: an extensive decal line that is worth looking at. I found it best to purchase one set to examine the sizes and determine how it can best be used. If judged useful, I go back and make a second order. As always, YMMV.