I didn't see that either, but you seem to be right. IMHO it should be D4 (Partial Plate 5 + Mounted -1 =4).
I agree with your point to differentiate the Burgundian Feudal Knights from the Imperials. With your suggestions there are also more ways of customizing these to your own intention.
I am aware, that I do not need your approval for house rules, but my intention is to improve the official lists/errata as well, so everyone might profit from these ideas. And for this purpose, you seem to be the authority^^