I wonder if there is someone that resolves the conflict with R. Yehuda that there are two "Tset HaKochavim" after all the Gemara in Shabat tells us that there is a difference between medium and small stars. If we resolve the conflict this way then "sunset" still remains the simple well known sunset when sun sets.
TIA
The BIUR HAGRA (O.C. 261) indeed resolves Tosfos' question with an explanation where sunset stays sunset. He explains that whenever we discuss Tzeis ha'Kochavim in Halachah, we really mean three quarters of a Mil after sunset. What does Rebbi Yehuda mean when he says in Pesachim that only after four Mil do the stars come out? He means that after this amount of time has passed after Shkiah, there is absolutely no light left, and all of the stars can now be seen, even the small ones that are difficult to see when there is some light still present.
All the best,
Yaakov Montrose