First - that's something you should be able to provide evidence for. 4Chan posts tag the date, so whoever identified them should be able to provide that evidence. Have you seen them repost it? If I were the person that had picked them out ahead of time, I would be pointing the earlier post with big bold letters. That this hasn't happened at this stage makes it less likely to have happened.
Second - It may actually be possible that they were picked out in those photos - there were A LOT of people picked out in the crowd by the likes of 4Chan, but the few that took off big time were
big time wrong. I'm talking
guy-having-to-plead-his-innocence-publicly wrong. I'm talking guy-
already-dead-wrong. If, amongst the many many people 4Chan and Reddit
incorrectly identified as the bombers someone, somewhere did actually pick out the real bombers, its not a sign of any prescience on their behalf.