By applying tar to the post you will help resist ground water from contacting the wood and speeding up the deterioration of the wood. 99% of posts deteriorate at the junction of the post and the top of the ground. This happens because that is the point where frequently water collects around the post and keeps it wet over extended periods of time. The use of tar helps keep the surface water off the wood surface, therefore tar should be applied to at least 3" above the ground surface. Tar all the way to the bottom of the post will also extend the post life.
Pebbles in the bottom and as fill around the post decreases the post life in that any surface or subsurface water will collect in the pebble void and remain there longer than if it were not there. There is no place for the free water to get away from the post until the soil around the post drys out.
The pebbles create a water collecting sump area without drainage. If you consider how a french drain works you will understand the object of the french drain, not a sump, is to move water along the french drain stone and not collect it for absorption. If water collects at the bottom of the post it will deteriorate from the bottom up, but it will take a very long time until it deteriorates a couple feet and causes issues.
If for any reason a treated post is cut off on either end the cut surface must be treated with a supplemental waterproofing or be protected with paint or a metal cap since the waterproofing material will only penetrate the wood approximately an inch leaving an untreated center. The top of a cut off post should also be sloped to shed water.
Concrete back fill establishes a more stable post and protects the post from water. After a period of time the joint between the post and the concrete frequently develops a crack and water runs down the post into the crack and deteriorates the post until the water can evaporate. Solution: caulk the crack when it appears to keep the water out of the joint.
If concrete is used, to make it stronger and less porous make the concrete as dense as possible. Use a rather dry mix and tamp it in place. Just throwing it in will result in voids and air pockets. The same applies with replacing the excavated material, install about 6" or 8" and tamp it in place, back fill it in lifts and tamp each in place. Bring the top of the back fill or concrete to about 2" or 3" above ground level to avoid ponding water around the post.
If you are building a 4' fence, a 6' post is just fine. 2' embedded and 4' above ground will provide adequate support and develop the full strength of the 4 x 4. If a 6 x 6 is used I would suggest using a longer post so that at least 2'-6" are underground to develop post strength. The bottom of the required post hole should be tamped whether using soil or concrete back fill to avoid voids under the post. If concrete is used the post hole should be over excavated 3" or 4" so that a concrete pad can be used under the post. Concrete should be placed in the hole before the post, then the concrete around the post can be installed and compacted. This operation should be accomplished in a continuous operation so that a cold joint, crack, is not created in the concrete at the bottom of the post.
The size of the post hole somewhat depends on the type of back fill. Concrete requires at least 3" cover on the post at its thinnest point. In that a 4 x 4's diagonal dimension is just over 5 and 5/8 inches a 1' diameter hole would be minimum. A 6" diameter hole would allow 3/16" post coverage which would crack quite rapidly. If the hole is back filled with soil the post hole should be a couple inches larger in diameter to allow space to adequately tamp the soil and compact it. The minimum hole size has been suggested they can obviously be larger which only results in more work and more materials. If you are in an area where the ground freezes and you are using concrete it is very important that the post hole diameter all the way to the bottom is the same or very slightly larger at the bottom. If the diameter of the concrete at the top of the post concrete in the hole is larger than the bottom the post will after few years of freezing and thawing push up and out of the ground. This thanks to a condition called frost heave. Even at the top of the ground, do not widen the concrete to make a wider surface than the post hole. It will either assist in lifting the post and at least it will break off at the first freeze. The top of the concrete should be sloped away from the post to shed water.
As far as the tar requirement it can be obtained at all building supply stores, roofing tar would work, asphaltic material such as used on asphalt pavement cracks and sealing would work, they also may have asphaltic paint available See what they have and consider their recommendations and the costs.
That's about all I can think of at this time, hope it helps in theory or reality.