The KA-3 and 4 (and for that matter the 1 and 2) are all great. They do their job and they do it well.
However, they don't compare to Railpro's PBM-2. The PBM-2 has circuitry that keeps the output voltage constant when voltage to the module is lost. This keeps the speed of the locomotive constant and the lights from flickering. Any other keep alive has an immediate drop in voltage which causes the locomotive to immediately move slower.
At some point I will make a video comparing the operation of the two types of keep alive. The results are quite dramatic.
-Kevin