As far as I was able to determine, 4.(H)/21 was stationed in East Prussia and Estonia. The Yellow band, if it is, seems odd in that it is so limited in its application. But then again the guy on the ladder might be just applying the yellow band and will expand its coverage.
4.(H)/21: Formed 1.39 in Stargard.
Disbanded 1.42.
Known bases: Stargard, 1.39 - 8.39 (He 45/He 46)
Gross-Lassewitz, 8.39 - 9.39
Pirmasens, 3.40 - 5.40 (Hs 126)
Octeville, 11.40
Memel, 6.41
Follin, 7.41
Jogeva, 7.41 - 8.41
Oesel(?), 10.41