The New King James Version uses the Tetragrammaton in the New Testament (written as +LORD [CAPS-smallcaps]) apparently it uses a Tetragrammaton source material, or it arbitrarily implies it while the KJV does not.
So, how does the Codex Sinaiticus refer to the Tetragrammaton within the Epistles of Paul? Specifically, an example is at Romans chapter 10 verse 13. [Paul is referencing Sacred Scripture]
For “whoever calls on the name of the +LORD shall be saved.”
+[Lord in [CAP-small-caps], indicating the Tetragrammaton was used as it's source]
[RSV] other translations treat our subject this way
For, “every one who calls upon the name of the ++Lord will be saved.”
++[Lord in [CAP-lowercase], indicating the Greek word "Kyrios" (meaning master or lord) is being used]
Roman's 10:13 quotes Joel 2:32, through Paul's letter
Here is Joel chapter 2 verse 32...
And it shall come to pass That whoever calls on the name of the +LORD Shall be saved. For in Mount Zion and in Jerusalem there shall be deliverance, As the +LORD has said, Among the remnant whom the +LORD calls.
And it shall come to pass that all who call upon the name of the +LORD shall be delivered; for in Mount Zion and in Jerusalem there shall be those who escape, as the +LORD has said, and among the survivors shall be those whom the +LORD calls.
And it shall come to pass, that whosoever shall call on the name of Jehovah shall be delivered; for in mount Zion and in Jerusalem there shall be those that escape, as Jehovah hath said, and among the remnant those whom Jehovah doth call.
Then whoever calls on the name of Yahweh will be saved. Those who escape will be on Mount Zion and in Jerusalem. Among the survivors will be those whom Yahweh calls, as Yahweh has promised.
But whosoever shall call on the Name of the Lord, shall be saved: for in mount Zion, and in Jerusalem shall be deliverance as the Lord hath said, and in the remnant, whom the Lord shall call.
So, the Question is how does the Codex Sinaiticus deal with the Tetragrammaton. Instead of trying to determine all cases, lets focus on Romans 10 verse 13. I suspect that it simply uses the Greek word "Kyrios" and in Greek letters. There was speculation that some early Greek Codex might have the Hebrew word which looks like the Greek Symbols Pi-Pi, and therefore it might possibly use two Greek Pi Pi letters, but not likely.