I was there and I, too, would guesstimate he's good for 3:54, maybe 3:53 now - just based on what I saw yesterday. Obviously I have no knowledge of what his training looks like.
He started in good position for the first lap, and I was pleasantly surprised to see him positioning himself well early. he employed Centrowitz-like tactics early on. The pace was slowish, however, and as others moved up, he allowed himself to get swallowed up a little bit, maybe falling back as far as 7th or 8th. The whole pack moved on the back stretch of the final lap, including Robbie, and with 200 to go I could tell he was shaping up for a big finish and that he would have a good shot to win, but I didn't think he was going to crush them like he did. I was about 30 meters from the finish trackside, and he looked like he was on repeat 5 or 6 of a series of 100 meter repeats. Very impressive the separation he got in the final straightaway.
However, the pace was not that fast. As stated above, whether or not he can do that off of a significantly higher pace is debatable. Certainly he's good enough to beat up on a lot of American runners and make easy money like yesterday. So, thanks to him for providing entertainment for fans like myself.