The rule is, apart from some inconsistencies with the existing mechanics, roughly fine. Basically, you're giving your players a new option to use, which is to their advantage - however, the option does have weaknesses that make it rather unattractive for frequent use. Here's some things I'd change:
It should be a free object interaction
You didn't specify whether throwing the potion counts as an action or not. Assuming it is, a word of caution: giving the potion (or any other item) normally (without throwing it) is a free object interaction (listed as an example on Player's Handbook, page 190), and has no risk of failure. Having to spend one's action and the recipient's reaction to throw a potion for those extra 20 feet, and with the additional risk of failure, seems like too steep a cost to me. Consider making the throw a free object interaction.
Neither attack or a saving throw is a perfect fit
Attack rolls are meant for, well, attacking, and don't work at all for situations like this - your party's monk would have a hard time catching the potion with their unarmored AC! Saving throws are meant for resisting harmful effects. Both are poor fits for situations where the two parties (the thrower and the catcher) are collaborating. The difference between a DC 15 ability check and a DC 15 saving throw isn't that big, but maintaining consistency with the core rules makes your house rules easier to remember and understand.
Note that thrown ranges like (20/60) only make sense for attack rolls. Since we're not using those, you'll have to use a different kind of range system.
Be mindful of Goblin Dice
"Goblin Dice" refer to a single die roll determining the outcome of things; sometimes unimportant like the life of a goblin minion, sometimes important to the point where you wouldn't generally want a single die roll to determine them.
The Potion of Healing is likely to be in the category of "too important for a single roll": its primary use in intense situations where one'd like to throw them instead of just handing them over is to serve as an emergency item. A loss of an emergency item, or even a delay in its delivery can be really detrimental to the party. I'd advise not leaving it up to a single die roll, or at very least toning down the effects of failure.
An example of a less painful failure effect is the potion landing safely on the ground, forcing the recipient to use their object interaction to pick it up.
Drop the critical fail
1d4 slashing damage is, in most situations, negligible, and in the situations where it isn't (eg. being down to your last hit point), it's likely to be extremely frustating. It's also worth noting that critical fails are not an official rule in DnD 5e - we're obviously talking about a house rule here, but if you haven't house ruled other saves/ability checks to have critical fails, I don't think the tiny bit of damage is a good place to start.
If you really like having critical fails in your game, consider having the potion breaking be the critical fail case, and landing on the ground intact the normal one. The loss of a potion is a nasty blow against the party's resources and shouldn't be overused, but critical fails are statistically rare enough to not deter using the throwing option too much.
You cannot choose to abstain from a potion effect
Potion of Healing states:
You regain hit points when you drink this potion.
In that language, there is no choice. Just cause and effect. You drink the potion, you regain hitpoints.
If you had a choice about potion effects, then the Potion of Poison would hold no threat.
This concoction[potion of poison] looks, smells, and tastes like a potion of healing or other beneficial potion...If you drink it, you take 3d6 poison damage...
If you could choose not to accept a potion's effects, then you could drink a Potion of Poison thinking it's one of Healing and then opt out of the effect.
Inability to decide anything
It's also important to note that the character is currently unconscious. Even if choice was possible, the only way they could make it is if they were conscious. Since they aren't, there definitely isn't a way for them to decline (just like how they can't decline drinking the potion.)
Best Answer
No, potions of healing don't hurt undead (unless you hit them really really hard with the vial).
Monsters and NPC usually have 1 healing surge per tier, and potions are one of the few ways they are able to tap on them. Some humanoid monsters from early monster manuals have the Second Wind power explicitly listed in their stat block (most template-created intelligent undead, for example).
The following healing potions (those that appear on the Player's Handbook) require the user to spend a healing surge in order to gain their effect.
The following potions, however, function even if the imbiber is bloodied and has no healing surges left. They can be found in the Mordenkainen's Magnificent Emporium.
Positive and negative energy is a thing of previous edition. In 4th edition, radiant and necrotic are damage type keywords, but they are not intrinsically tied to the healing keyword or mechanics.
For example, some undead have healing powers (like the Skull Lord, the Spirit Devourer, or the Voidsoul Specter).
The way you handled the thing was, in my opinion, correct. You gave the hint to your players that the healing keyword does not mean radiant damage.
Edit: In a previous version of this answer I stated that monsters usually have no healing surge. This is false, as @wax eagle pointed out.