Are the 4 cameras which are 1.5km away all near each other? If so, I would install a second DVR there, then connect that via fiber, instead of converting each channel individually. I would tend to think a single fiber run with standard fiber media converters and a second DVR would present fewer potential problems than fiber transceivers which, at least for HDTVI, are a less mature product with limited suppliers.
If they aren't near each other, transceivers may be a good backup plan. Do you have transceivers in mind? I haven't seen many available.