Is this bulb actually dimmable?

Yes, it is rated dimmable. It needs a dimmer designed for LED loads; older incandescent-only dimmers can cause flicker or buzzing.

How bright is a 4W LED?

4W LED is roughly comparable to a 25–40W incandescent, so treat it as accent or ambient light rather than a main room light.

What fixtures does a T10 bulb fit?

T10 is a tubular bulb about 1.25 inches across and longer than a standard A19. It suits exposed-bulb pendants, sconces, vanity bars and chandeliers with open sockets.

What color temperature does it come in?

That depends on the specific version you pick; check the color temperature on the bulb you order, since warm white and daylight versions look very different.

Can I use it outdoors?

Only in a fixture that keeps it dry and sheltered. The clear glass and open socket are not made for exposed wet locations.

Is it worth it over an incandescent T10?

Yes for running cost and life, and it dims without the heat of incandescent. If you want maximum brightness from a small bulb, a higher-wattage LED is the better pick.