A know enough a lot of very good bands with 4 and less albums in their catalog. I especially like the following ones:
UK (2 albums + 1 live) (I think it needn't comments)
Locanda Delle Fate (2+1) (One of the nicest bands ever! Unfortunately haven't heard their latest album)
Asia Minor (2, excellent "French" Turks!)
Cai (3, Flamenco-Progressive from Spain!)
Taproban (2, but they are the new band - I hope these Italians will release records further)
Tibet (1, outstanding Germans!)
Al-Bird/X-Religion (2, Heroes of Progressive from Uzbekistan!)
Chris Squire (1, Haven't you forgotten about his 1975 album? )
Gracious! (2, one of the earliest mature Proggers from UK)
Pentacle (1, so beautiful band from France with the only wonderful album in 1975)
Aphrodite's Child (3, the third one (666) is essential album!)
I Giganti (2, brave Italians!)
Morple (1, another brave Italians!)
Jumbo (3, and another brave Italians yet!)
Ache (4, their 1st release (1970) contains two excellent side-long tracks, the second album is very good too, didn't hear their latest albums though)
Anglagard (2+1, I think they needn't comments)
Sinkadus (2+1, another very good band from Sweden)
Tangent (2, being not a big fan of this supergroup, I admit that it's very good anyway)
Blind Faith (1, one of the 1st superbands ever )
Cro Magnon (3, for those who likes Avant Garde at its best!)
U Totem (2, their 1s album is the unique case of the Genre I may call as "Sympho-Avant-Garde"! The 2nd one is made in jazzy way, but very good too)
Well, maybe I'll get to remember some bands yet...