The kid is 4. So, its probably going to take a few years to figure out either the correct set of dxes, or the complete set of dxes.
This is just MY guess, but...
If anything is likely to stick in the long run, it would be the ADHD diagnosis. Can it be noticable at 4? Sure. AND he's responding to medications. So, either it really is ADHD, OR the real diagnosis is one of the broad-spectrum dxes that includes lots of ADHD traits. Either way, go with this one for now, learn all you can about it. (Try: Driven to Distraction, a book about ADHD that's really well done) Other than medications (which work with some people and not with others), there's lots of well-tried ideas out there - and again, not all of them will work for your child, but there's good stuff out there. Schools are also (usually) somewhat aware of things that work for these kids.
Would be interesting to know which tests they ran to determine the ADHD... and whether or not they tested executive functions... because that is likely to be part of what you are dealing with - these include things like inhibit (self-control), initiate (getting started on stuff), planning, shifting (changing activities), etc.
ODD is kind of a catch-all diagnosis that more often than not gets tossed out to label problem behaviors that they don't have another explanation for. Yes, there are behavior problems... you already knew that! Why? this doesn't tell you anything more. MIGHT give you a little leverage at school.
PTSD??? not impossible, but... what kind of a trigger? at age 4? That would be a therapist/neuropsychologist determination.