Usually, this would mean burning out your tubes. If you are driving the system hard and not paying attention, you could do more damage to the amp. Your speakers should be fine unless you are at 100% volume when somethings goes....
Honestly though, if you watch the tubes to make sure they aren't burning/glowing too hot, you can try the 8 ohm taps to hear the difference. The 8 ohm taps may get a very slightly warmer sound?
It's not worth the trouble though. Just use the 4 ohm taps and all should be great. I have amps with 1.4 ohm taps that don't care what they drive (2ohm, 4, 6, 8, 16) as long as it isn't less than 1.4.
I like your gear and don't feel that you are losing anything by using 4 ohms taps. That's what they are there for!
