The other day I saw a new light put in a yard. It's either a fluorescent or HPS fixture, and they actually mounted it to a 2x4 for a post. 

I'm wondering how well it will stand up, especially if it gets windy, like in a hurricane. I guess we shall see.

I don't think that's gonna hold it well but it's anyone's guess. Steel at the minimum should be used. I used to have a pole outside my shed, that was taken down due to the building getting taken down, It was made out of some 50x50x3mm steel box and was 4m tall. It held up pretty well even in a few high winds I was a bit worried about it falling over but it held nicely. It was very sturdy so much that I could stand on it without it falling over but that's because I bolted it to a retaining wall.

I used to have a 175 watt mercury yardblaster I installed in my parent's back yard on a 4x4 post. I installed it in a hole maybe 4 feet deep. Before that I had a temporary setup with a 2x4 in a 5 gallon bucket filled with sand for stability. Apparently it wasn't that stable because I came home from school one day and it had been knocked over and the reflector/refractor was damaged.

I don't think that's gonna hold it well but it's anyone's guess. Steel at the minimum should be used. I used to have a pole outside my shed, that was taken down due to the building getting taken down, It was made out of some 50x50x3mm steel box and was 4m tall. It held up pretty well even in a few high winds I was a bit worried about it falling over but it held nicely. It was very sturdy so much that I could stand on it without it falling over but that's because I bolted it to a retaining wall.

Steel is massive overkill for something like this. A regular 4x4 post works just fine in an application like this. You just need to make sure it's buried deep enough.

One of my lights , a Westinghouse OV-10 is mounted to a 16' 4x4 bolted to the side of my garage .  It's been there for almost 10 years .  Depending on the weight of the aforementioned light (a yard blaster I presume ?) , how deep the 2x4 is sunk into the ground , how tall the 2x4 "pole" is , the condition of the 2x4 , it COULD actually last awhile . HOWEVER , in a hurricane , all bets are off !  If a hurricane comes MY way , all of my lights are coming DOWN  .

One of my lights , a Westinghouse OV-10 is mounted to a 16' 4x4 bolted to the side of my garage .  It's been there for almost 10 years .  Depending on the weight of the aforementioned light (a yard blaster I presume ?) , how deep the 2x4 is sunk into the ground , how tall the 2x4 "pole" is , the condition of the 2x4 , it COULD actually last awhile . HOWEVER , in a hurricane , all bets are off !  If a hurricane comes MY way , all of my lights are coming DOWN  .

Im pretty sure hurricanes do not hit in ohio 

Actually , we got the remnants of hurricane Ike in 2008 . It caused MASSIVE amounts of wind damage ! The damage totaled more than $1.25 BILLION  just in Ohio !

Some "rednecks" in the village not far from here use a row of 2x4's to carry an overhead to their roadside saloon (probably from another building deeper inside the village) - They look so completely wacky and all askew, but they have been up for like 5 years now and still going.... (i dont know with how many collapses and repairs during this time, but i dont see any odd splices in the cable)

(They probably did this because the entire structure does not have any building permit, so the power company refuses to give them service)

Actually , we got the remnants of hurricane Ike in 2008 . It caused MASSIVE amounts of wind damage ! The damage totaled more than $1.25 BILLION  just in Ohio !

Oh. WEll i can understand why you want to take down all the light then.

One of my lights , a Westinghouse OV-10 is mounted to a 16' 4x4 bolted to the side of my garage .  It's been there for almost 10 years .  Depending on the weight of the aforementioned light (a yard blaster I presume ?) , how deep the 2x4 is sunk into the ground , how tall the 2x4 "pole" is , the condition of the 2x4 , it COULD actually last awhile . HOWEVER , in a hurricane , all bets are off !  If a hurricane comes MY way , all of my lights are coming DOWN  .

Yes, it a yardblaster with acrylic refractor.

I was actually thinking to use a 4x4 or 6x6 to mount an HPS NEMA in my yard, if my parents let me. Somewhere mostly out of view though.