I didn't see the price on this horse, and I don't know what your price range is, but I don't think he's a realistic prospect to reach 4th/PSG.
He's 11, not been in consisent work and has some holes in his foundation work. He's a realistic 1st/2nd level prospect. With good training and maintenence, maybe 3rd, but as anabel pointed out, you'll be fighting a battle of attrition with age and miles by the time you get him there. And I also agree with anabel that he'll struggle with the collected work based on his confo.
I liked him, though - seems like a good guy and a willing worker. If the price is right, and you'd be happy with something that would top out at lower than PSG, he still might be worth a look. I would be asking lots of probing questions about why he hasn't progressed further in his training by his age, though.