Updated for air patches: Expand me for the chart of destruction:
Disclaimer - AFKers were harmed in the making of this video.
Q: How many C4 does it take to destroy X tank at full health?
A: All of the tanks in the game, lightning, prowler, magrider, and vanguard, die to two C4 bricks.
Q: Are the quadbikes buggy?
A: Yes, C4 planted on a quadbike may or may not do damage when rammed into a tank. This is a BUG. When it works, 2 C4 on a quad will kill any tank.
Q: As a light assault, do I have enough fuel with the default, un-upgraded jetpack, to plant 2 C4 on a tank and detonate it from a safe distance before falling?
Prowler tank -
can be destroyed by 2 C4 bricks planted on any location on the tank. 2 C4 bricks off of the tank and nearby can also destroy the prowler, but will need to be extremely close. This is in effect regardless of the prowler being in deployed or undeployed mode. A single C4 brick will destroy between 2/3 and 3/4 of a prowler tank's health.
Vanguard tank and magrider tank share the statistics of the prowler. C4 placed on the ground that a magrider is hovering over / passing over will also still destroy it, but like the prowler, require 2 C4 bricks.
A lightning tank will lose just shy of 3/4 of its health to one C4 brick. If it is damaged, there's a decent chance you'll be able to destroy it with a single C4 brick. All tanks will lose over half of their health to one C4 brick, so visual recognition of the damage indicators may inform you to its vulnerability.
The vehicles in the game suffer some location based damage, but the values of this are irrelivant to someone planting C4. No matter where you plant your C4, two bricks will kill it, and one will not, regardless of C4 on the front armor or on the rear.
---Ramming vehicles with a quadbike with 2 C4 bricks on it ---
in the video I tested C4 on a quadbike closer to a lightning than in a prior test, and it didn't kill it. This demonstrates that, as the C4 were closer when not killing and farther when killing, that this is in fact a bug, and you should be able to place 2 C4 bricks on any quadbike and ram it into a tank to destroy it - now that it is fixed. Sortof. It doesn't ALWAYS fail.
---- What if I come across an enemy pilot repairing his vehicle? ---- OUTDATED ---
1 C4 brick will deal about 85% of the health of a reaver, mosquito, or scythe, when planted on the frame of the vehicle - slightly less if planted on the wing. 2 C4 will destroy any one-man air vehicle.
UPDATE: One C4 brick will kill one-man fighters
2 C4 bricks will take off just shy of half the health of a liberator bomber. OUTDATED---
UPDATE: Two C4 bricks will do almost enough damage to make a liberator catch fire, but not quite
3 C4 bricks will destroy a sunderer. OUTDATED -
SUNDY PATCH NOTE (NOW OUTDATED) - 2 c4 bricks kill a sundy if placed on the back unless it has explosive resistance. What determines this explosive resistance, I am not sure. Sometimes it takes me 2 to kill one at full health, sometimes it takes me 3.
SUNDY PATCH NOTE 2 - Two C4 bricks will take a sunderer down to burning health if it is at full and doesn't have armor upgrades where the C4 was detonated. However, attacking a sunderer with a grenade launcher, or if it has taken one rocket hit, or simply taken damage from bumping into something (and these scenarios are more common than uncommon) will allow 2 C4 to kill the sundy. I find this to be the case with most sunderers, as odds are they will have bumped into something along the way before deploying, or have taken at least one rocket. Or, if you're LA or engineer, a grenade launcher round.
SUNDY PATCH NOTE 3 - Grenade launcher rounds now do less damage to sunderers and a combo of 2 C4 and a grenade round will not destroy a sunderer, but will still make it burn, unless C4-resistance certifications have been added.
Galaxy - PATCH NOTE - C4 was bugged, now two C4 ontop of a galaxy will do around half of the galaxy's health. Not sure about damage from the sides. ---OUTDATED---
UPDATE - Galaxies now require 4-5 C4 to destroy depending on placement. Too many light assaults were killing galaxies that were slightly damaged and too many unskilled pilots were hovering, so C4 was nerfed to cater to idiots.
---What if I want to ram a one-man aircraft into a tank?---
Currently does not do damage unless aircraft ram other aircraft. Possibly still buggy post-launch.
UPDATE - aircraft-to-ground ramming is still, many months later, still buggy.