At least one official US Army Air Service sub-type designation can be added to the list above - the DH-4C.
DH-4C - 1921 DH-4B modified by Packard for its 1A-1237 V-12
- DH-4C: Atlantic Aircraft (Fokker)-built DH-4B conversion; x 1
- DH-4C: Needed new engine mounts, cowling & radiator shell
-- Wing position shifted to allow for c/g change; new cabanes
A few of the surviving DH-4s also received later USAAS role-letter designations when modified. Some examples are the two types listing in the X-CO (Experimental - Corps Observation) category:
CO-7 - Boeing modernization of de Havilland DH-4 2-seater, x 3
- X-CO-7s: DH-4M-1* fuselages; new main u/c & tapered wings**
- X-CO-7 : 1924; USAAS serial A.S.24453 (c/n 520, ex-A.S.22884)
-- XCO-7 airframe delivered without engine fitted; static testing
- X-CO-7A: 1924; A.S.24453 (c/n 519, ex-A.S.23109 as DH-4M-1)
- X-CO-7A: As XCO-7A except delivered with standard Liberty 12
- X-CO-7B: 1925 A.S.24454 (c/n 521, ex-A.S.31216 as DH-4M-1)
- X-CO-7B: Fitted with inverted Liberty 12-A; balanced elevators
--
https://1000aircraftphotos.com/Contributions/GeorgeNelson/5192.htm
-- * DH-4M-1 was Model 16 within Boeing's designation system
-- * However, X-CO-7B was assigned internal Boeing Model 42
-- ** 2-bay panels with tapered planform outboard of first bay
-- CO-7 changes gauged "very poor" by USAAS observation board
CO-8 - Atlantic-built DH-4M-2 fitted with Loening CAO-1 wings, x 1
- X-CO-8: 1 x 420 hp Liberty 12A engine, 2-bay wing span 13.72 m
- X-CO-8: Atlantic built DH-4M-2 A.S.23163 (Wright Field's P-369)*
-- * Wing-swap seems intended to extend service life of DH-4Ms**
-- ** As opposed to testing for Loeing; if so, failed in its objective
An X DH-4M-1 of 1923 is also mentioned but doesn't fit the USAAS designation style. This is a bit of puzzle. As a Boeing-assembled airframe, X DH-4M-1 might have been an internal, corporate designation. But Boeing normally assigned its own model numbers - eg: Boeing Model 16 for the DH-4M and DH-4M-1.
There were also a range of DH-4s modified for trials purposes which seem to have had no designation changes at all. Some examples are:
DH-4s fitted, as per X-CO-8, with Loeing wings panels (x 20) for airmail service
Engineering Division ambulance: DH-4 mod.; 2 x litters + doctor & pilot
-- Wikipedia lists as 'DH-4Amb' [[i[sic[/i]]; I highly doubt this to be official
DH-4B modified as carrier for Engineering Division GL-1 target glider
-- DH-4B upper wing mods/various cradle types for launching of GL-1
--
https://www.secretprojects.co.uk/threads/aeromarine-projects-and-prototypes.18424/page-3#post-266851
Doubtless there are heaps more ...