4 ohm vs 8 ohm What is the danger ?

You could let the smoke out of your equipment.... This happened to me with a low end receiver on some high end speakers 2-4 ohm operating impedance... The protection was intact (rear panel fuse), the other protection that caused the unit to be silent would take effect from time to time, but the end result was an early death of the output fets. The only reason this setup was even attempted was my need for music while my amp was out for an upgrade... This may not occur in every circumstance, but I am unlucky enough to find out the consequences. Luckily this unit was under warranty and was replaced for no cost to myself...

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.