Here it is: your Phantasy Star IV missable item list!

[etony33]

For the purposes of this list, there are three separate categories.

-Category 1 is something that is only accessible in a chest in an area that cannot be revisited after a certain point in the game)
-Category 2 is something that is equipped to a character at the time when they join or rejoin your party, making it actually the opposite of missable, but if the item is sold or otherwise discarded, you can never get another one
-Category 3 is something that can only be found in a chest, but the area can be visited at any time up until endgame. However, if this item is sold or otherwise discarded, you can never get another one.
-Category 3.5 has one single item: the Psycho Wand. It cannot be replaced. However, it cannot be sold or discarded. You can get rid of it, however, by giving it to Raja before triggering the events which lead to him leaving your party.

A hypothetical Category 4 would include Elsydeon and the five rings, as these items are all one of a kind, but there's no way to get rid of them.

These are all listed in alphabetical order. I've almost certainly made a few mistakes, so please feel free to correct me if you notice one. And, without further ado:

Cat 1:
Laser Barrier* (There are two. One is Cat 1 (Zio's Fort), one is Cat 3 (Plate System))
Laser Sword (Zio's Fort)
Power Shield (Garuberk Tower)
Swift Helm (Air Castle)

Cat 2:
Ceramic Gear* (There are two. One is Cat 2 (Wren is equipped initially), one is Cat 3 (Nurvus)
Cyber Suit (Seth is equipped initially)
Head Gear (Demi is equipped initially)
Impacter (Demi is equipped initially)
Laconian Axe (Gryz is equipped after getting Elsydeon)
Laconian Armor* (There are two. One is Cat 2 (Demi is equipped after getting Elsydeon), one is Cat 3 (Vahal Fort)
Laconian Gear* (There are two. One is Cat 2 (Demi is equipped after getting Elsydeon), one is Cat 3 (Vahal Fort)
Mahlay Dagger (Hahn is equipped after getting Elsydeon)
Mahlay Mail (Hahn is equipped after getting Elsydeon)
Mahlay Ring (Hahn is equipped after getting Elsydeon)
Mahlay Shield (Hahn is equipped after getting Elsydeon)
Plasma Rifle (Wren is equipped initially)
Psy Robe (Rune is equipped when he rejoins in Ladea Tower)
Spaced Armor* (There are two. One is Cat 2 (Wren is equipped initially), one is Cat 3 (Nurvus)
Titanium Armor (Demi is equipped initially)
White Mantle (Rune is equipped initially)

Cat 3:
Ceramic Armor (Plate System)
Ceramic Gear* (There are two. One is Cat 2 (Wren is equipped initially), one is Cat 3 (Nurvus)
Compo Armor (Climate Center)
Compo Gear (Climate Center)
Elastic Armor (Weapons Center)
Elastic Gear (Weapons Center)
Frade Mantle (Ladea Tower)
Guard Armor (Courage Tower)
Guard Claw (Two, both in Strength Tower)
Guard Mail (Two, one in Courage Tower, one in Anger Tower)
Guard Robe (Strength Tower)
Guard Rod (Strength Tower)
Guard Shield (Silence Tower)
Guard Sword (Silence Tower)
Laconian Armor* (There are two. One is Cat 2 (Demi is equipped after getting Elsydeon), one is Cat 3 (Vahal Fort)
Laconian Gear* (There are two. One is Cat 2 (Demi is equipped after getting Elsydeon), one is Cat 3 (Vahal Fort)
Laser Axe (Plate System)
Laser Barrier* (There are two. One is Cat 1 (Zio's Fort), one is Cat 3 (Plate System))
Laser Knife (Plate System)
Laser Slasher (Chest in Kadary)
Napalm Shot (Kuran)
Photon Eraser (Vahal Fort)
Plasma Field (Zelan)
Plasma Launch (Weapons Center)
Plasma Sword (Zelan)
Pulse Laser (Zelan)
Pulse Vulcan (Climate Center)
Silver Tusk (Myst Vale)
Spaced Armor* (There are two. One is Cat 2 (Wren is equipped initially), one is Cat 3 (Nurvus)
Stun Shot (Plate System)
Titanium Gear (Plate System)
Wave Shot (Nurvus)
Zirconian Armor (Kuran)
Zirconian Gear (Hangar)

[Zucca]

Surprisingly many Laser -items. O.o
Also some Ceramic ones I thought could be bought.

[etony33]

A lot of it is android equipment, which cannot be bought anywhere (Ceramic Gear and Ceramic Armor). The other Ceramic stuff can be bought.

[Doctor Hooves]

A lot of Plasma stuff too.

I guess shops can't find out how their mechanics work, since laser utilities in swords do seem a bit too advanced.

Though laconian (I still have no clue what that means) can be easily be found in shops in Dezoris.

[etony33]

I guess shops can't find out how their mechanics work, since laser utilities in swords do seem a bit too advanced.

They knew the technology 2000 years prior, and also 1000 years prior. Perhaps they forgot that information during the Great Collapse.

[Drake_Zeppeli]

Ah, the Power Shield, reminds me of my latest playthrough of PSIV and I was trying to find it, but then I accidentally found Dark Force 2.

[etony33]

I'll try to come up with a "perfect inventory" list later today. It would involve having every item (other than story items like the Canceller and Eclipse Torch) either in your inventory or accessible (i.e. in a chest that you can get to after getting Elsydeon), with every character having allvof their best gear equipped (there's a bit of wiggle room here - for example, Chaz, Rika, and Gryz all can equip the Guard Mail, but there are only two).

[etony33]

What would Rune and Raja's ultimate equipment be?

It'd be the Dezo Ring for Rune, but beyond that, it's tough to say? Two Psy Shields? Silver Shields? One Psy Shield and one Laser Barrier? Guard Rod? Psycho Wand (which has very low attack power but is Factor 6)? Who gets the Guard Robe?

[etony33]

Here's what a "perfect inventory" would look like in my opinion.

Chaz: Algo Ring, Elsydeon, Cyber Suit
Rika: Mota Ring, Silver Tusk, Guard Claw (or Genocyde Claw depending on enemies), Guard Mail
Rune: Dezo Ring, Psy Shield, Psy Shield, Reflect Robe
Wren: Palma Ring, Pulse Vulcan (or Photon Eraser depending on enemies), Laconian Armor

Hahn: Mahlay Ring, Mahlay Shield, Plasma Field, Mahlay Mail
Gryz: Laconian Helmet, Defeat Axe, Guard Mail
Demi: Laconian Gear, Sonic Buster, Laconian Armor
Raja: Laconian Circlet, Psy Shield, Psy Shield, Guard Robe
Kyra: Laconian Crown, Moonslasher, Moonslasher, Reflect Robe

Landrover, Psycho Wand, Ice Digger, Hydrofoil, Aero Prism, Rykros Ring, Laser Barrier, Laser Sword, Power Shield, Swift Helm, Ceramic Gear, Head Gear, Impacter, Laconian Axe, Plasma Rifle, Psy Robe, Spaced Armor, White Mantle, Guard Claw, Guard Claw or Genocyde Claw, Guard Rod, Laconian Rod, Pulse Vulcan or Photon Eraser, Mahlay Dagger

All Cat 3 chests must be accessible, whether in your inventory or still unopened. And yes, this would mean unequipping Demi of everything before fighting Zio.

[Gav the Monster]

Wouldn’t it make sense to give Hahn the Mahlay Dagger? He’s fine with just one shield.

I’d give the Plasma Field to Kyra. I don’t think she needs two Moon Slashers, since it would be useful for freeing up space since the Moon Slasher isn’t missable.

I’d also give the Psycho-Wand to Rune and the Guard Rod to Raja. I doubt they even need shields once they’re at a certain point.