How many 'Great Tales' were there?

[gondowe]

This is the passage I refered to from the Letter Nº131, He was talking about the Tale of Beren and Lúthien:

There are other stories almost equally full in treatment, and equally
independent and yet linked to the general history. There is the Children of
Húrin, the tragic tale of Túrin Turambar and his sister Níniel – of which
Túrin is the hero: a figure that might be said (by people who like that sort of
thing, though it is not very useful) to be derived from elements in Sigurd the
Volsung, Oedipus, and the Finnish Kullervo. There is the Fall of Gondolin:
the chief Elvish stronghold. And the tale, or tales, of Earendil the
Wanderer. He is important as the person who brings the Silmarillion
to its end, and as providing in his offspring the main links to and persons in
the tales of later Ages.

Greetings

[Galin]

Maybe you're thinking of a note to The Shibboleth of Feanor.

"As is seen in the Silmarillion. This is not an Eldarin title or work. It is a compilation, probably made in Numenor, which includes (in prose) the four great tales or lays of the heroes of the Atani, of which "The Children of Hurin" was probably composed already in Beleriand in the First Age, but necessarily is preceded by an account of Feanor and his making of the Silmarils. All however are "Mannish" works."

JRRT, note 17.

[Mithadan]

The four tales are also discussed by CRRT in the introduction to Children of Hurin. He cited correspondence to Waldman though not by name if I recall. The 4 were Beren and Luthien, Hurin and Turin, Tuor and the fall of Gondolin, and the “voyages” of Earendil.