You Samaritans worship what you do not know; we worship what we do know, for salvation is from the Jews.
The rejection by the Samaritans of the prophets, the psalms, and the historical books greatly limited their knowledge of God. They knew Who to worship, but they didn’t know Who they were worshipping. The Jews were given the greater revelation of God and thus had the capacity to know who they were worshipping. Their problem was that they did not take advantage of their greater knowledge; they didn’t use what they had.
Jesus said “we” because He was a Jew.
“salvation” is literally “the salvation”. Jesus is probably referring to the salvation that comes through the Messiah. The Messiah is a Jew.