I ahd the same issue 13 days into owning the S4, bog standard phone with no routing, it was a little intermittent (as faults always are when yor trying to show the phone shop) but the forward facing camera never seemed to have a problem working, I ended up asking for a replacement handset there and then as is was still within 14 days. new handset and not a problem since.
The shop did try to say it was down to a app causing the problem till they went through all aps installed and couldn't find anything. The only camera app other than standard i had tried was snap chat but only had it on the phone for a day and removed all the associated files.