Hi Thommy
I dont think that Lt dR Heinrich Büssing (P) ever was in Kest 4a. Instead he joined Kest 4b in early summer of 1917 after leaving Jasta 5.
Kest 4a and Kest 4b was created when "old" Kest 4 was split up in two units on 15 Apr 17.
The location at that time for Kest 4b was Freiburg in Br., a nice town I have visited several times...
On 3 Oct 18 Kest 4b was renamed to Jasta 84w. I dont have any known observers serving in Kest 4b...sot it was basically an one-seater unit.
Kest 4a was stationed mostly at Böblingen near Stuttgart.
Oswald Boelcke was never in any Kest, but he served in the Verdun sector in spring - early summer of 1916.
Do you have any more names, then I maybe can help you identfying units..?

Gunnar