The question is whether that amp can run bridged into 4 Ohms, and my guess is it probably won't like it. That might be why he said that, and you are correct in that in bridged mode, the output is higher though the impedance internally seen by the amp is already lowered by running it that way. In other words, in bridged mode, you are limited to 8 Ohms. The way the amp delivers power is affected by the impedance of the speaker; lower impedance will draw more current from the amp, making it work harder. In bridged mode, you cut that in half because the two channels are tied together into one, so the amp is already working harder.
It is hard to say if you will damage the driver; it really depends on how loud you have it cranked, and whether or not you are playing it to the point of clipping. If not, it should be fine short term, but long term, I'd get a separate amp for it. If you are driving it to clipping, it is only a matter of time before something fries.
