Yes it is very safe to run on your Buzz at 4v, your Buzz does have IC protection for over temp, over current, and reverse battery. I'm pretty sure your Buzz Pro goes as high as 15W (might want to check with Notcigs) so you can go as high as 4.7v with a 1.5 Ohm Carto, at least you could on Version 1. I haven't had a chance to look at the specs on the newer Version 2.
Depending on the the Tank size 3.5 or 6, with 1.5 mls being in the Carto you should be able to put 40+ mls through your Carto before needing to clean or replace it. You can clean the Carto and get another 20 mls or so through before it's toast.
You'll know when you will need replacing or cleaning because
juice will build up in the Air Wick Tube and start gurgling. So when that happens stop topping off the tank, vape whatever is left in there, then replace or clean the carto.
Hope that helped....