From my decath experience, in my best competition for the 100 and the 1500 I ran 10.96 and 4:50. I came into decath with a throws background, and I don't think I'd run over three-to-four miles in my life. I had only had a couple years of formal running training, and rarely ran more than 500 m at a time.
Greene's 100 m time was about 12% faster than mine, so even if that "fast-twitch disadvantage" completely works against him, making him 12% slower over the mile, that'd put him under 6 for the mile. That assumes, however, that fast-twitch doesn't help you at all in the mile, which isn't true.
The other factors, like superior running economy, better distance-running body type, greater career training volume, not having contested 9 events previously, etc., all indicate that he could run faster than 5-mid. Since no decath guys who are 10-mid-ish break 4:30 for the 1500, I'd say 4:40 would be preposterously low, but in the 5-range sounds very reasonable.
Now, if you put him in a 10k I'd agree with some of the people who predict preposterously slow times. Just from my experience, I hopped in a road race out of college because I was rooming with some distance guys, just to answer a speculative question like this, and it wasn't pretty.