Hi Justine,
i am sorry to hear that, it may be nothing, i know some hamsters have two nests sometimes if they think it would be easier to cope that way, but not usually with only 4 babies.
The babies are only appox. 6 days old at the moment, so unfortunately there isnt anything you can do if she is leaving one baby out, and there certainly isnt anything you can do if the baby is already dead, or been destroyed.
Usually after 6 days of everything being well, the only reason a mother would abandon or destroy just one baby would be becasue it was ill or it had died on its own - and that might explain why she has moved the others away too.
Please dont worry, nature usually knows best and it will all sot itself out for the best.
Good Luck.
Tammy x