Lighting – light fixture wattage question

AFAIK, bulbs with a GU-10 base are driven directly from electrical mains, in which case it's OK to use a lower-power bulb. Your fixture won't reach the temperatures that it's capable of withstanding.

The only place that I know of that it might not be OK is with low-voltage (usually 12V or 24V) lighting systems such as track or task lighting: some transformers have a minimum power output that they like to supply, so if you use lower wattage bulbs, you should use more of them to compensate (somewhat defeating the purpose). If this applies, you would have an entry for Minimum power on the specifications for the lighting system.

This is a bad idea.

The maximum power rating for low voltage lights is lower than that for normal lights because no transformer is 100% efficient, so some allowance is made for that inefficiency in the specifications. In your case, those four lights draw 200 W plus whatever overhead the transformers add, so you're drawing more current than the switch is designed to handle. More current through the switch means more waste heat generated by the electronics in the switch, which leads to higher temperatures inside the electrical box, which in turn will shorten the life of the switch¹.

To answer your question about the brightness: I'd expect that they'd get less current than they should normally which would make their light both dimmer and redder. Generally, you have bright lighting in bathrooms, so even if it weren't a problem for the switch, it might not be something you'd want.

^{1. If you look at the specifications for the switch, you may see de-rating information for using two or more of them in a single electrical box; this de-rating is caused by the waste heat generated by the switch.}