If I successfully teleport a creature 4 square above another creature, and it falls, what happens? Is damage done to one or both?
[RPG] What happens when a creature falls on another creature
damagednd-4efalling
Related Solutions
The best relation is the Falling Roof Trap listed in the DMG and Phandelver Module. If triggered, it deals 4d10 bludgeoning damage, but allows for a DEX save. Here, you're dropping something significantly larger, from a greater height.
You can reach back further to 3.5 and compare it against the Siege Engines. A heavy Catapult launches a sizable stone for 6d6 bludgeoning after traveling an arc with a base of at least 100 ft. Again, this is something larger, falling from a greater height.
The math for this starts with the understanding the surface area of the impact to determine the DC for a dex save, a perception check with a decreasing DC to perceive the falling man, and an estimation of the mass of the falling object. For a base assumption, refer to page 249 of the DMG for Improvising Damage. This is less than a crashing flying fortress, but more than compacting walls. In terms of Damage Severity, this should be at least deadly for any one caught in the center of the target area, and dangerous for those on the edge.
There's quite a few different scenarios here, let's deal with them one at a time:
1. Does a rider take falling damage?
It would depend on the height of your mount but most mounts are not 10 ft tall so you wouldn't take falling damage.
You quoted the relevant rule:
At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. (SRD p.86)
Most mounts are classed as large creatures. A large creature could be up to 10ft by 10ft in size (SRD. p. 92). However a commonsense ruling is that large creatures are simply bigger than medium sized ones (5 ft by 5 ft) and don't take up the whole square. In any case it's unlikely that a rider would fall from the tallest point (it's head) of even an unusually tall 10 foot horse rather than it's back (which will always be a point lower than ten feet).
As it is unusual for a mount to be taller than 10ft no additional damage is normally taken.
2. If the mount is knocked prone is the rider also knocked prone?
If you don't take any damage from the fall, under the general rule a PC wouldn't be prone.
The creature lands prone, unless it avoids taking damage from the fall. (SRD p. 86)
However this is a case of specific beats general:
If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it. (SRD p. 99)
If you choose not to use your reaction, or cannot use it you are knocked prone despite taking no falling damage.
3. If the mount is forcibly moved against the rider's will are they knocked prone?
If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. (SRD p. 99)
In this instance the rider can make a saving throw to stay mounted, but once again specific beats general, so if they fail the saving throw they fall prone even if the don't take any fall damage.
4. What if the rider is knocked prone while on top of the mount?
Despite being prone they have the option to not actually fall off the mount:
If you’re knocked prone while mounted, you must make the same saving throw [as previous quote]. (SRD p. 99)
If they pass they're 'prone' on the mount, if they fail then they're prone on the floor.
Being prone on the mount may sound odd but there's nothing in the prone condition that states it can only occur on the floor.
5. What about mounts taller than 10ft?
The same rules as above apply - only this time you will also take falling damage, which would knock you prone either way.
6. Of course, all of this is worse if it's a flying mount...
The rules as above apply - you just risk taking a lot more falling damage and either way you'll be prone.
It'll be more the height at which you're flying that's the issue than the size of the mount you've fallen off of (though both are relevant). If you presume that the player is sitting on top of their mount (rather than hanging on underneath it) then the fall will begin from the top of the mount and not below it - so falling distance would equal the height of the mount above the ground plus the from which you fell on the mount itself.
Best Answer
Note: Remember that 4e has a set of rather simplified, abstract rules to cover only situations that come up often. For such rare occurences the GM needs to improvise and make up something new based on the ruleset. With that in mind you can read below.
As Colin said the creature falling would certainly take the regular amount of damage. As for the other one, creatures generally don't occupy 100% of their grid space. That is quite apparent since realistically you can fit a few average people in a 5x5 feet square. So, it wouldn't be a certain hit on the creature under the falling one.
I think you should roll something versus the Reflex of the creature-standing-below or have it make a Dexterity check to dodge the falling object/creature. Relevant size between that creature and the one falling should affect the DC/roll as well, since dodging a halfling should be easier than dodging an Adult Dragon that somehow ended falling from a few feet above your head.
For instance:
Something similar can be done if you want to roll vs. Reflex instead of having the target roll a Dexterity check.
Regarding damage in case of a direct hit, there are two variables to consider, mass of the falling object and falling distance (momentum gained from greater height would make the impact more intense, just like it works for the falling damage).
For damage due to mass you can either set an amount of damage per weight unit (lb) or creature size category, the latter aiming towards simplicity (eg 1d10 per size category over Tiny, or 1d10 per 100 lbs).
For the falling distance, you should should set an amount per square fallen, or even use the formula for falling damage.
In a more realistic sense, I think I would like to use some sort of multiplication between these two factors however that would be easier to keep balanced and would take a few skills in maths and differential analysis to make sure you don't go "unreasonably off the chart".
All in all, you need to make up something. Good luck with that.