Jay,
The 4509 and Q4509 have the same power requirement and should be
interchangeable. Did you check the Q4509 bulb to make sure that you
did not get a bad one? There is no reason that Q4509 can not be used
in place of 4509.

We have used both 4509 and Q4509 for landing/taxi lights as well the
Galaxy brands also ordered from Aircraft Srpuce which are much cheaper

11-02219 GALAXY 4509 LAND.LIGHT BULB $7.95
11-02220 GALAXY 4509Q HALOGEN LAND LGT $19.85

The 4509 is rated for 25 operating hours vs 100hrs for the Q4509.
Shortly after we bought the plane, both the taxi light and landing
light burned out. We contacted the previous owner for information on
what bulbs to get and he told us in all 9 years of owning the plane and
flying hundreds of hours, he had never changed any bulbs. He recalled
flying about 5hrs at night in all those years. It's most likely that
he did not turn the lights on except at night time. One of the
instructors introduced Rick to the idea of living the landing light on
ALL THE TIME for safety reason. This is the source of contention for us
for a while until we switched to the long lasting quartz light. Rick
reasons that $10 or so every 25hrs, the light bulbs are cheap insurance
against collision. My counter argument is that leaving them on all the
time will increase the chance that the darn bulbs are burned out when
you really need them. So our compromise is that we will use the Q4509
and Rick will only turn on one light (not both) during takeoffs,
landing and enroute in poor visibility or at night.

Yes, I have read seen reports that one can use NAPA's tractor bulb
equivalent but since the Galaxy 4509 is even cheaper than autopart
store price, why bother?

Hai Longworth