4ft deep should be plenty if you are in Florida, UNLESS you have exceptionally loose soil. Schedule 40 pipe is typically used. 3.5" quoted is typically the INSIDE diameter of the pipe. The outside would then be 4". You'll need at least 800lbs of cement, so 10, 80lb bags, or 20, 40lb bags.
It's FAR better to "overkill" in this case. Cement is cheap. It's easy to dig a hole, make the hole "bell shaped" (smaller on top, and bell-out at the bottom. If you make a perfect cylinder, the wind MAY spin the whole thing! Dump a bag of gravel on the bottom, helps with ground water drainage, so it doesn't lift your pole. Place a nice flat paver on center top of the gravel, (keeps the pole from sinking before the cement sets up completely) Set the pole on the middle of the paver, AFTER you have drilled a hole all the way through the pole (at least 1 ft up from the bottom) and shove a chunk of all-thread through it. Use some 2 x 4's screwed to wooden stakes (driven into the ground outside the hole) to level and stabilize the pole North/South AND East/West. You can decide how to connect the 2 x 4's to the pole. I use U-bolts, but even GOOD duct tape can work.
Once you are POSITIVE the pole is perfectly level towards the TOP (where the polar mount goes) it's time to mix and pour the cement. Do it evenly, and slowly on all sides of the pole. Personally, I let it dry at least 4-5 days before I mount my dish, but I have very sandy-loamy soil. I also check it a few more times with the level during the first few hours. If the pole somehow slips/leans and hardens, you're screwed. If it slips and you catch it in time, you MAY be able to fix it by simply yanking it straight. Once you think it's set up, remove the 2 x 4's and stakes so the pole is all by itself.
