Risk of overloading a light fitting?
Since these are individual 120V bulbs rather than a transformer/driver with low voltage DC bulbs, the rating is based on heat. It could be the design can dissipate 6W without a problem but the certification (hopefully UL or similar) was based on tests at 4W. Only the manufacturer of the fixture could honestly tell you if 6W is OK.

My hunch is that unless this is a totally enclosed fixture that 6W would be fine. But I can't guarantee that. There are two risks:

Minor: Shorter bulb life
Major: Fire

Not worth the risk. 5 minute test no problem but leaving on unattended has a small but very real risk. If there were a fire the your insurance company would not be happy to find a 6W bulb in a 4W fixture.