4 in. x 4 in. x 42 in. In-Ground Fence Post Decay Protection

Yes, mine were closed on bottom

You could but if you leave open at the bottom for some drainage. Otherwise the post will rot because it sits in standing water. Other things to think about: It is best to have 1/3 of the post below ground and 2/3 above ground. For added strength you could do 1/2 below and 1/2 above. Also for a corner and end post use a 6"x6" or for a long run even an 8"x8". As they are much stronger and will last longer.

I didn’t see a need in filling up the sleeves, but I did caulk around the top to keep water out

Yes. I have one installed this way for the past 3 years and it works well. I set it on a 6" stone backfill base with 3-4" of stone backfill around. Drill some water drain holes. I also wrapped the post with 1-2 layers of cardboard so it would really "fill out" the post protector, thus ensuring the pressure of the stone and earth wouldn't compress the protector and make it too hard to take out later.

I have 4x4 cedar posts that I will use with these 4x4 Post Protectors. My posts will fit into these Protectors just fine. A little coaxing will be necessary as the fit is snug.

Like all plastic it has some give.

The bottom is sealed, however the installer has to drill 1/8" holes to allow any water that comes in from the top to escape. Put gravel or stones in the bottom of the hole to prevent water build up in the hole.

Application dependent. The reason I say this is I put one in the ground fire clematis, so I followed the directions and drilled drainage holes at the bottom. I now have to put one on a block. I thought at first I may not need to drill the holes, but I am going to do it. You may want to or not want to.

That would be a question for the manufacturer. I do not feel it would but manufacturer would know best

Yes. It is similar to the plastic in Milk containers. Any wood saw will easily cut through it. For fences, you should allow at least 4 - 6" to extend above the ground, anticipating ground debris accumulation