I know that it says "affixed to the hooves of a horse or similar creature", but would it count towards Centaurs, since you know... Lower half horse?

I don't see why it couldn't count. Centaurs have hooves. As do Minotaur. 

The horse shoe items were intended for PC mounts. And npc beings like centaurs and such
They were never balanced for a PC themselves to use. So therefore i doubt they could be (especially since the centaur PC race is not the same size as a proper horse or even centaur. As they are medium not large)

Ravnica centaurs are medium for the purpose of game balance and exploration, lest they wield 2d10 glaives and cannot squeeze through small openings like other PCs.

It's also mentioned in the DMG that you need appropriate appendages to use certain magic items, so such a character cannot use boots of elvenkind.

Don't magic items reshape to fit the creature that attuned to it? Then again, this item doesn't require attunement...

Chapter 7 of the DMG, under Magic Items (specifically Wearing and Wielding Items and Paired Items) states that appropriate appendages are necessary to wear or use magic items and that paired items (in this case, a set of 4 horseshoes) need to be worn together to gain the benefits.

Yeah I think I’d allow a centaur pc to wear them but have it take up an attunement slot. 

my DM in Odyssey to split a set between me(satyr rogue) and the minotaur barbarian....needs to be attunement or he's gonna be pissed at us soon in this campaign 

since a PC wearing these horseshoes would be floating above the ground, they would not/could not make any noise when walking. So... similar to boots of the elvenkind: While you wear these boots, your steps make no sound, regardless of the surface you are moving across. ... and therefore advantage on stealth?

I get the point about Centaur not having 4 hooves, but what about Triceratops that have 3 hooves by frontlimbs and 4 by backlimbs???

Just curious but wouldn't the creature require protection from heat if crossing above lava? I understand that they avoid burning from direct contact but proximity to magma is still enough to cause trees and other materials eg houses to spontaneously combust. So a dread mount or paladin's war horse using theses would still need protection from heat/fire to cross the river of lava threatening the damsel/dandy in distress and for which the rescuers will receive a handsome reward.

Also, as a homebrew rule and if allowed in my campaign, I'd say that any hooved creature could use these as long as the requisite number of shoes were available. So a satyr with only 2 hooves would be fine and would have the extra set if the one pair got damaged - the same for the minotaur. On the other hand, the Sleipner would need to 2 sets - 8 shoes to gain the benefit. In other words the number of hooves is irrelevant as long as there are sufficient shoes for each hoof. And I agree a centaur can't wear a boot but definitely can use these shoes. - Provided there's a competent smith available to attach the shoes to the hooves. Once affixed I would rule that they remain affixed until purposely removed unlike regular horse shoes which are known to come loose at any given time just because of wear and tear or riding through rough hard ground with lots of stones and rocks and such.

Can these be applied to a steel defender, as long as the steel defender has hooves? Going to change my bear to a Deer. 

Also, how does the floating 4 inches interact with falling from a height? Would my steel defender be immune to fall damage, or atleast be able to fall without major issue? 

While the flavor is different, the effect is nearly identical to a Ring of Water Walking (Uncommon, no attunement). The biggest differences ignoring difficult terrain & the 50% travel time increase. I'd say those are more than compensated by the increased rarity of the item.

Quote from SparkyMark >>

Can these be applied to a steel defender, as long as the steel defender has hooves? Going to change my bear to a Deer. 

Also, how does the floating 4 inches interact with falling from a height? Would my steel defender be immune to fall damage, or atleast be able to fall without major issue? 

 Is it a "similar creature" to a horse? (i.e. does it have 4 legs with hooves that can accommodate horseshoes.) Then yes!
The item doesn't say anything about falling damage, so RAW nothing changes. Personally I'd flavor it as making little platforms below each hoof, as opposed to levitating.

Paladins mount.... Lets go!

how long would it take to make a set of 4? How much would each cost? I would like to have an entire squadron of floating paladins 

Very Rare items typically cost 50,000 GP from stores. But XGtE puts down crafting costs at 20,000 GP and a whopping 25 workweeks, plus any number of magical ingredients to make the items.