Well, I really wasn’t sure other than that the category 4 is rated for more demanding conditions. So I did some research and dug into IEC 61010-1 and found that category 3 is rated for most uses OTHER than outdoor utility connections and category 4 meters are rated for all uses.
There are also some voltage considerations and limitations to the different categories but the primary difference is not the regular duty but the high voltage transients. High voltage transients are often called “surges” or spikes and are most likely when working on outdoor transformers and distribution panels.
Rubber meets the road is that for HVAC use a category 3 meter is likely going to do the job but if you ever work in main panels, or outdoor transformers go for a cat 4 meter.
PS – Fluke has a great info sheet on this HERE
You can see more about the Testo 760 shown HERE