Hi Johan,
I'm certainly not an expert, but I'm great at quoting others' research!!
Most sources agree that Collin claimed what he called a Bristol at 0930 hrs on 23 May, which he said fell at Maria-Alter, SE of Brügge. I'll unashamedly steal from Rick Duiven's research, and tell you that his updated Jasta 56 listings agree with you that this was the DH4. In the
The Jasta War Chronology Collin's victory is identified as the DH9 D5616, in "Over the Front" it is also identified as a DH9 from 27 Sqn.
The "DH4/DH9 File" by Ray Sturtivant and Gordon Page says for DH4 A7840: "Left 0610 for raid on Thourout, in combat with E.A., went into dive Maria Aeltre (sic) -Capt. AE Palfreyman killed and 2/Lt WI Crawford POW wounded.
For DH9 D5616, the same source says, "Left 0610, bombing raid on Maria Aeltre (sic), engine fell off. crashed Sains-lè-Pernes, wrecked. Lt GR French and Cpl F Y MacLauchlan both killed."
I would assume that these planes were both on the same raid, since they left at the same time. Since the report submitted by their squadron mates says that the DH4 fell after a combat with EA. and the DH9's engine simply fell off

, I would agree with you that the DH4 was probably Collin's victory. As far as I can determine (leeching off Rick and my other sources again) Collin was only credited with one victory, and no one from Jasta 56 or any other units made significant claims in this area either.
I'm sure other here have more info than I do. I knew the DH9 could be a lousy airplane, but the engine falling off?? I wonder if FLAK made any claims.
Greg