x1 is a foster parent of x2 instead of previous parent/rights holder x3 until x4
x3 might be the bioparent if alive, other adoptive family, or the government if the child is a ward of the state or an orphan. x4 is the next step, for example reunification when x3 completes some legal requirement (e.g. drug rehab), or permancy if the foster parent is expected to adopt the child.