If you had asked me which Beatles would be hooked on heroin, I would have told you George Harrison.  Harrison had that hollow-cheeked look you often see from users.

How did Lennon manage to get involved in heroin while the other three lads not?

George and Paul had what George called "a built-in safety switch" that kept him from getting too heavily involved in drugs. Like anybody, they had bad periods-- Paul after the breakup of the Beatles and George after his marriage unraveled. But they rallied and went on to be relatively sober all their lives. (Poor Ringo succumbed to alcohol, but also rallied in time-- good for him!)

I think John enjoyed living on the edge, and heroin was an exciting high. It must have been, for so many people to succumb to it. So, if you take what Paul said about LSD and applied it to acid, "John welcomed that experience whereas I tended to shy away from it."

In addition, George watched two of his closest friends lose themselves for a time to heroin: John and Eric Clapton. There were death tolls a-plenty, but the withdrawl into a personal world of these two must have hurt the most. After his Bangla-Desh concert, when Dick Cavett asks George about heroin, he says, "I didn't want to know." I think seeing what happened to others was repellent to him rather than drawing his curiosity.

for a guy who went "cold turkey" in the early 70's (presumably 1970, which I think is when he quit using), he looked great in his 1971 video for "Imagine."