Question: I am confused about references to Kelim and Baba Metzia, and I can't find the text in either tractate. The citation I have is: Kelim Baba Metzia Chapter 4 Halacha 3.
Answer: This is a citation to the Baba Metzia "section" of tractate Kelim of Tosefta. Perhaps you are not finding the citation because you tried looking in the Mishna (Tractate Kelim in Seder Tohorot, or Tractate Baba Metzia in Seder Nezikin), or in the Talmud...
Because of its large size, Tosefta Kelim is divided into three sections: Kelim Baba Kama, Kelim Baba Metzia and Kelim Baba Batra (first gate, middle gate and final gate). Each of these sections is divided into perakim (chapters), which in turn are divided into halachot (laws).
These three sections of tractate Kelim in Seder Tohorot should not be confused with the three tractates in Seder Nezikin, also called Baba Kama, Baba Metzia and Baba Batra. Scholars theorize that there had originally been a tractate Nezikin in Seder Nezikin, which had been divided into these three sections, but the overarching title had become eclipsed with time, and the sections became tractates in their own right.
Kelim is the only tractate of the Tosefta with this type of tripartate "gate" arrangement. This arrangement has not survived into the Mishna, which divides tractate Kelim directly into chapters and mishnayot like all the other tractates of the Mishna. There is no Gemera for Kelim.