It's really really common to wake up between 3-4 at night for many many people, with and without PTSD (or other mental disorders). From what I understand (when I researched it a long while back) it has something to do with our hardcoded circadian rhythms and nightly metabolism.
I've woken up somewhere between 3:30 and 4ish without fail almost every day for pretty much two decades (unless I'm really really really physically exhausted), ever since I was a teenager. Whether or not I also had this before, I don't remember, so I don't want to claim whether or not it started in adolescence or way earlier. But it was/is so consistent, I could literally tell you the time without even looking at a clock. However, I don't have trouble falling back asleep and don't get up and actually *do* anything.
Like you, I too at some point made the connection with trauma. My trauma happened around 4am (when I was 13). It's a logical conclusion. And I do think it plays a big part in this, at least for me. But I did want to mention that it's really not uncommon. I feel like I remember
@somerandomguy mentioned something along those lines somewhere at some point. I also know my boss (no trauma/PTSD) often wakes up at 4am, but like you (and unlike me) he's also *up* then.
To answer
@joeylittle 's question: for me it's independent from when I went to bed/fell asleep. Also independent from any seasonal influences (subtle changes in outdoor ambient light in the summer with its early mornings, for example)
ETA: Keep in mind that there are genetic morning people and genetic night owls. It's a thing. Our personal circadian rhythms are hardcoded and very very very hard to actually change (without any health impacts). I would assume a genetic morning person waking up at 4 is more likely to be up for the day than a night owl who'd naturally sleep till 8-10 if they could. I'm the latter, so I don't think it's surprising that, while I do wake up consistently around that time, I also have no problem falling right back asleep, while my boss for example is a genetic morning person.