People who think the baby is Jaime's: Jaime, Tyrion, Cersei
CERSEI lifts her hands from the chair and places them both on her stomach indicating she's pregnant. Jaime looks shocked.
JAIME: Who will you say is the father?
Cersei cradles her belly and Tyrion notices. She takes a deep breath.
CERSEI: That thing you dragged here, I know what it is, I know what it means. And when it came at me, I didn't think about the world. Not at all. As soon as it opened its mouth, the world disappeared for me, right down its black throat. All I could think about was keeping those gnashing teeth away from the ones who matter most, away from my family. Maybe Euron Greyjoy had the right idea. Get on a boat, take those who matter.
TYRION: You're pregnant.
People who think the baby is Euron's: Euron, Cersei, Qyburn
So when Tyrion talks about the baby in S8E4, he means Jamie's and Cersei's baby. He does not know about the Euron+Cersei "baby". He has no idea about the Euron baby as Euron, Qyburn and Cersei discuss it just moments after Danaerys' boats are ambushed by Eurons fleet. And there is no way this information would have reached Varys and Tyrion so quickly. Also if he knew the baby's was Eurons, we can safely assume he wouldn't care that much as opposed to the baby being Jamie and Cersei's.
When Tyrion goes up to Cersei to tell her to consider things with her baby in her mind, he's doing one of the two things:
- Genuinely thinking for the good of Jamie and Cersei's baby and advising her to think of the baby.
- Knows how Cersei keeps her men bound to her by lying and manipulation. Cleverly throwing in a baby shade aiming at Euron, to get into his head and if possible turn him against Cersei by talking about about Cersei and Jamie's baby. So that if Euron knows about a baby, he may back off support to Cersei. Tyrion knows his sister and probably knows Euron is in the Cersei basket for the sleaziness of it.
People who actually know if the baby is real or a thrones move: Cersei and Qyburn.