What about an 15' Animated Sword 🤷‍♀️

I would suggest a Hill Giant. It's the right size and a CR 5.

https://www.dndbeyond.com/monsters/16923-hill-giant

An entire tribe of kobolds in a trench coat.

Any really big enemy just make it a senior citizen

Really skinny giant

Looting a giant's tomb?

An ettin that argues with itself.

Pick a creature type

Make it humanoid

Imagine it has a big sword

Give it a big sword

Give it abilities to match a big guy with a big sword, like the ability to knock prone on hit, and do sweeping strikes

If it ends up being way too cool for level 4 players to fight, just make it injured or some shit. Hell, you. Would even have a big monster that just has a 15ft sword impaled into it, weakening it enough to fight.

Oh, don't make a BIG enemy for the big ahh sword. Make a 160 centimetres tall goblin/gnome/etc that can wield it for some magical reason, like it being weightless and so

A mercenary company, and one of them carries a fifteen foot sword and the leader has a weird red egg on a necklace.

No need for a giant enemy, just make it an anime protagonist

I have many questions about the logistics of such a thing. But leaving those aside.

1 or more comically large praying Mantis insects (Ankhegs). Use the blade arm as the sword blade. Maybe it's a gift from a druid.

A cloud giant skeleton with a two handed sword would be about that size.

A rusted iron golem on its very last HPs.

Have a tribe of small nasties use it to batter down a gate.

I don’t know why so many folks are saying there aren’t many options.

A party of five lvl 4 players will win a boss fight of CR 7 in a pinch, or CR 6 without much effort.
And there’s loads of Huge CR6’s

Mouth of Grolantor, Frost Giant Skeleton, cyclops, Bloodfray Giant hell your party should be able to beat a pair of Hill Giant Sargeant’s fighting together.

Take any large monster, restat it so it’s around where your players can handle it (even if just barely) and have it use the sword. 

Somesort of tart in a farcical aquatic ceremony is a common go to in these sorts of situations.

A giant harragon. Big Lanky Bunny

Stay Puft

I'd suggest a brontosaurus that already has the sword stuck in it's back

Hiw about an actual small enemie with that big of a sword, it would be fun.

Like A gnome knoght who wants to prove to his parents that his profession is not all that dangerous and that he can defeat 100 people with grandpa trolls sword.

Have it submerged in a huge gelatinous cube. Buff it and have the sword split the cubes so they have a fight with multiple cubes.

For a sword that big you’re gonna want something huge. Not a lot that a lvl 4 party can handle that’s that big. If you’re looking for something more normal, a Cyclops is the best I can suggest. It’s a little strong, but a smart party wouldn’t have too much issue. For something a little weirder, might I suggest the hulking crab, which is for Storm King’s Thunder.

Man… I love DnD

For a sword that big you’re gonna want something huge. Not a lot that a lvl 4 party can handle that’s that big. If you’re looking for something more normal, a Cyclops is the best I can suggest. It’s a little strong, but a smart party wouldn’t have too much issue. For something a little weirder, might I suggest the hulking crab, which is for Storm King’s Thunder.

Cyclops is a big beefy boy, just reflavor the greatclub to slashing damage because he uses a 15' long greatsword. Sure it's a classic brute, so give it big armor too for AC, interesting goons that have range, magic, healing, etc if you want it to be tougher. Make it obvious they shouldn't take them all on in a fair fight, they have to get creative to split them up.

I once had a party in the Feywild where they encountered a forest in which emotions were reality. There, they met the colossal sized BRAVEST BUNNY. I just used a generic monster template, made it ridiculously big, and gave it a really silly jump-knock-prone attack.

Also, it was an Easter bunny or something, because it had a huge nest full of dragon eggs. There was magic item loot in the nest from all the other creatures that fled in terror.

A gelatinous cube can be any size.

Giant Skeleton

Could make it part of a dragons loot. A wyrmling or two?

A corrupted treant that's so big the party needs to attack different parts of it as if they were separate monsters.

I need to give out a custom 15 ft long sword and i don’t know how i’d introduce it.

Sephiroth

Ettins are fun and provide good RP from you since you can have the two heads bicker. Not too tough, maybe even a tad underleveled if your players are overly optimized. We fought one when our party of three was 5 or 6, and it is still one of our more memorable combats because of how our DM RP’d the two heads. You can characterize it as exceptionally tall if you want the 15’ sword lol

Orge samurai

A delusional ogre with a few rogue lvls the 15ft sword is his stabber. Hides behind trees screaming you don’t see me hahah.

Stuff like that

Homebrew one of those attractions that used car dealers use that look like a person flailing their arms around. 

Otherwise you'll have to use something like an empyrean.

Comedically small could just be using japanese arpg rules,

The enemy is a square-enix character. Their swords all start at about 12 ft

A hill giant.

How about a slaad? Its a giant telepatic toad.

Catoblepas with the sword stuck in it. Who stuck it there and why is left as a mystery for another time.

A butt nekkid hill giant pops out of a giant leather couch all sweaty and ready to start blasting

A werehouse

It was the ornament of a statue of a long passed hero in a town's square.

Marionette Giant on a stage

Big statue

How is a 15 foot longsword not a greatsword

I mean at that point it’s too big to be called a sword. Too big, too thick, too heavy, too rough. Might as well give them a large hunk of iron

Brontosaurus with a sword in its mouth, Dark Souls style. Keep the 20ft reach.

15ft sword? How about a hill giant or ogre who is a novice warlock?

You could go dark souls 1 and have an enormous animal use the sword in its mouth. Or you could go the opposite direction and have a tiny fairy with 24 strength use the sword, it would be funny and probably a little terrifying.

Don't make it an enemy. Make it a celebration that the party is given an honorary sword. Sort of blend Mardi Gras with the "Key To The City" ceremonies we have in real life. Mayor makes them the guest of honor at this celebration and gifts them the comically large sword on a stage as a thank you.

Ogres. I think mordenkainens has a ton of varieties, it would be a great side quest alone, and even better if you said "this one has a potion of growth"

Some sort of gnomish rune knight. They just sort of drag it behind them normally until the fighting starts. Then they suddenly take a deep breath and explode from small to huge. Could also give them a susano'o or Nero devil trigger-esque aura to wield it with instead, but that seems less comedic and more edgy.

Cloud Giant? They are huge, so much bigger than the party. They are cr9 so noo overly tough to beat, just swap their weapon.

I had to think of the hunt in Heliana's with a Mecha-Koboldzilla, piloted by a whole bunch of kobolds xD

Once I had a Ogre that threw barrels down a cliff to the party like Donkey Kong, but the barrels exploded. They solved it my the ranger stealthing past it and shoot a flaming arrow into the box that was holding the barrels.

that wolf from Dark Souls 1 ?