I would measure the voltage between the two hot lugs at the panel if its a single phase panel, or all 3 lugs if its a 3 phase panel. If you get around 208 volts your ok. Anything over 215 volts means its not a 120/208 volt 3 phase utility supply to the building.
208 volt heaters are created for buildings with a 3 phase 120/208Y services. A 240 volt heater will produce much less heat when connected to a 208 volt supply. A 4000 watt 240 volt heater will only run at about 3000 watts at 208 volts, a 25% reduction. To solve this problem manufacturers offer 208 volt rated heaters. However, not all 208 volt heaters can handle 240 volts, since at 240 volts a 4000 watt 208 volt rated heater will produce 5,325 watts. Some cant handle the excessive heat hence the sticker.
If curious about the voltages on the sticker and where you find them:
240 volts usually comes from the typical 120/240 single phase service found in most homes and some apartments. Less likely is a delta high leg service that gives both 120/208 and 240 volts. (other services can give 240 such as 240/480 split phase; 138/240Y; 240 ungrounded delta ext, ext but those apply to industrial environments and not worth worrying about)
208 volts comes from 3 phase 120/208Y services. Its the norm in commercial, a lot of apartments and houses in cities. Often individual apartment units and houses only get 2 legs and a single phase panel but the 2 hots read 208 instead of 240. 120 volts is still found hot to neutral.
277 volts comes from 277/480Y services. Very common in larger commercial, industrial and high rise buildings. 277 volts runs commercial lighting, heaters; while 480 volts runs motors and elevators. 480 volts is also stepped down to 120/240 or 120/208 via transformer for outlets and everything else you would normally find running on 120 volts. Not something most HIs will encounter in residential settings.
Long reply but hope it helps.