No danger normally. A 4 ohm speaker will want to draw twice the current from your amp as an 8 ohm speaker. If your amp is not a high current design then this could result in distortion. Generally a bright sound that hardens into glar as the volume goes up. I own Thiels that have impedences of less then 4 ohms over most of their bandwidth. It I don't use a very good amp they sound very harsh. I use a totally current unlimited amp ie one that doubles its power from 8 to 4 ohms and doubles its power again from 4 to 2 ohms. Its a Krell and some say you can arc weld with it. If you run your speakers very loud for a long time you might trip the breakers in your amp or blow a fuse but you're not likely to do it any perminent damage.
