Imperial 4 in. x 4 in. Outdoor Black Cast Aluminum LED Solar Post Cap (2-Pack)

yes with the included attachment

No these will not fit 6X6.

Mine are bright, clear white, not yellowish.

These are shiny black and match very well with an aluminum black fence

Yes. Switch goes Automatic or Off.

Absolutely. The caps fit my 4x4 post perfectly so I haven't bothered to nail them down. They have been up two years and all the adverse weather (including high winds) have not affected them. I am so pleased with my solar post caps and will be purchasing several more as soon as I complete my next project. The lights are still burning brightly at night and they add so much class to my deck.

Included screws are small, I used one inch black with a small black Phillips head on them and they fitted flush when screwed into wood.

The included screws are small, maybe quarter inch or less. They barely secured to my posts without a tapering.

That is a good idea. Here's something for you to consider. My contractor installed and instead of making them even on post, he made the lights all the same height. You don't really notice the post differences (it is usually only a small difference). What this installation method does is keep the lights level. When they are on at night they look awesome. Bright and lights all the same height. (he used a lazer measurement tool) I have 11 on my back porch installed on wood 4x4 post. With metal fence between. The light matches the metal fence really well.

these are made for a 4 x 4 post.