Steve, Sandy,
possible but note a retired officer wore the half moon in the opposite colour to those of active officers. So in all likelihood a Rittmeister z.D. or a.D. of DR 4 with permission to wear the uniform on retirement; "Bekleidungsvorschrift" 1899/1911, page 162, paragraph 163 b).
Landwehr-Kavallerie officers wore epaulettes with a field in the colour of the infantry regiments of the corps with the Arabic number of the corps in gold; see the "Bekleidungsvorschrift" 1899/1911, page 160. The 5, 6, 16 & 17 Corps wore yellow, hence no 4 on a yellow field.
Regards
Glenn