If I recall correctly, III & IV's are wideband only. But, it seems that they will still receive just fine, just lower output audio from what I have read.
Correct, neither is designed to be narrowband. A lot of how they will perform in a NB environment is dependent on how well the new transmitters are tuned, and especially how their audio levels and deviations are set.
Some folks are reporting mediocre performance and others report good performance. There's a bit of "wait and see" to it all.
I have heard that there are shops offering to rework III's and IV's to NB, presumably by swapping out some components and retuning them. Prices have been quoted between $90 to $150 per pager. I am not sure I would sink that much money into an older pager but others are doing so. And no one to date has actually given me contact info for any of these places so it may all be rumor anyway.
III's & IV's are showing up in increasing quantities on Ebay and other sale sites so there are some good bargains to be had. I would just urge caution until you know how one will perform in your local narrowbanded systems.