“At least three readings should be read from the Old Testament, both from the Law and from the Prophets, and their respective Responsorial Psalms should be sung. Never, moreover, should the reading of chapter 14 of Exodus with its canticle be omitted.” (Roman Missal, Easter Vigil, n. 21).
“In place of the Responsorial Psalm a period of sacred silence may be observed in which case the pause after Let us pray is omitted.” (Roman Missal, Easter Vigil, n. 23).
The canticle for the “reading of chapter 14 of Exodus” is from chapter 15 of Exodus. So in a sense it is another reading, even though it is a Responsorial Psalm.
The Alleluia before the Gospel is a Responsorial Psalm, using Psalm 117. I regard it as also required.
So if Responsorial Psalms are counted as “readings” then a minimum of five Old Testament “readings” are required for the Easter Vigil.