Which element corresponds to the configuration [Xe] 6s^2 4f^4?

The electron configuration [Xe] 6s^2 4f^4 indicates that the element has a total of 6 principal energy levels filled up to xenon, plus an additional two electrons in the 6s subshell and four electrons in the 4f subshell.

In the context of the f-block elements, which include the lanthanides (elements with a partially filled 4f subshell), the configuration specifies that we are dealing with an element from this series. The presence of four f electrons indicates that we should be looking at the element with an atomic number that corresponds to the filling of the 4f subshell.

The 4f subshell begins filling after lanthanum and cerium, with neodymium filling the 4f subshell with 3 electrons (4f^3). As we move through the lanthanides, the electron count increases sequentially. When reaching the element samarium, it has 4f^5 configuration. Thus, for 4f^4, we must look one element before samarium in the sequence, which corresponds to europium (Eu), where the configuration would be [Xe] 6s^2 4f^7, and