Ummm, not really, but thanks all the same 
I should probably explain that I don’t know if it’s exactly 4.5 mm or an inch size that’s somewhere between 4 and 5 mm. So, to rephrase the question: does anyone who actually owns or has owned a TRBX304 (cough @howard cough) know the exact hex key size needed to adjust the truss rod?
Not sure how this relates to the TRBX304 
it’s metric, 4mm. The three wrenches you should use with the bass are 4mm, 2mm, and 1.5mm.
edit: just checked the service manual to verify and those are the wrenches it ships with.
Thanks for your reply, @howard. I tried my 4 mm hex key and it fits inside the truss rod bolt (or whatever it is called), but it doesn’t “grip” it. It just rotates inside the little hole; the truss rod itself stays in place. Guess one of the previous owners must have demolished it. I’ll try it once more, but I may have to improvise.
Thanks also for the service manual. I was looking for something like that on the Yamaha website, but I couldn’t find it 
Ouch. It’s possible they stripped it, yeah.
There’s also an outer fitting and an inner bolt BTW, can be hard to be sure it’s seated right.
Well, I gave it another try, and it turns out I was wrong. The 4 mm hex key fits just fine, and the truss rod turns without any problems. It just did so without any resistance at all. Now I have to admit this is the first time I’ve ever tried to adjust a truss rod, and I was expecting it to take a considerable amount of force. As it turns out, it was already all the way loose. When I tried turning it clockwise, it started getting tighter and it took a little more force to turn it. So it appears to work as it should.
So now the question is: with the truss rod all the way loose, the neck is pretty much dead straight, as far as I can tell. I lowered the action quite a bit last week, and I’m getting a little bit of fret buzz when playing the higher frets (12 and up). So I wanted to loosen the truss rod to get a little more relief, but that turns out to be impossible. Guess I’ll need to raise the action a bit more… But first I’ll replace the strings with the stock D’Addario EXL170’s.
Yeah it’s possible for the truss rod to be completely loose. As long as it tightens and straightens the neck eventually, it’s all good.
How much neck relief currently exists? If you fret the E string first fret and the fret at the body joint, how much gap at the 7th-9th frets are we talking?
Not much, but I’ll have to check it. Might take a while though… It’s my daughter’s birthday tomorrow, and people are coming over both this afternoon and tomorrow to celebrate 