@ Alayta
That's a difficult story. There are plenty of district twins consisting of a proper urban district and a rural district surrounding the former, both sharing the same district code. That's of course nothing new to you, but as I used to live in two such "holes in the donut" and for a short time even in the donut itself, I'm more confronted to the subject.
Your four-digit objection can be explained easily. Combinations with two letters and four digits first come into use when the old space up to two letters and three digits is going to deplete. As I know from "my" district twins, the rural district usually gets the room with two letters and three digits. The urban district gets the rest, meaning everything with only one letter and up to four digits and everything with up to two digits. If the city is to big and the donut is to slim, it may also be the other way, but generally it's been like I said.
Current situation is ambiguous. As I said, the old limited space is more or less depleted which leads to interesting arrangements how to divide the XX-0000 room between the various twin pairs. Normaly both sides get an equal share nowadays. On the other hand, the rural districts are lucky that the new falsification-hindering scripts allows for registering letters that used to be to ambiguous to have them registered with the old DIN script.
Many twin pairs have e.g. arranged that single B, F and G letters are to be exclusively used by the rural districts when in combination with four digits. So R-B 2237 might well belong to a suburban dweller of Regensburg rural district.
True, Königsberg, before and after the ware, usually had about 400,000 inhabitants, that's what I didn't remind. But I think that Königsberg would still have a sizable suburbia in this scenario like any city. And I tend to think that the Rhine-Neckar district (which carries HD for Heidelberg) is more populated than Mannheim and Heidelberg combined. So I think that the plate is OK for a Königsberger sururbian as well.
When IOTL the Poles switched to a similar system after reintroducing three-tier government (wojewodztwo - powiat - gmina), they made it better in some way. E.g. the City of Wroclaw is carring DW (Dolnoslaskie - Wroclaw), but Wroclaw Country is carring DWR. OK, therefore we have LL instead of L for Leipziger Land and LAU instead of N for Nürnberger Land.