What does this strange symbol refer to?
Strictly speaking, it is not a full circuit, but an equivalent full circuit can be easily derived from it, because it has the full information. It has two "labels" (which are not an actual components), which are denoted with the GND symbol (the bottom one) and the triangle symbol (the top one). Both are denoting the voltage levels of the nets they are connected two. The ground is denoting the voltage of 0V (reference potential, to be precise), and the triangle is the denoted (+4V) voltage relative to that reference. The equivalent circuit would look exactly like this one but with a 4V power supply with "+" connected to the triangle, and "-" connected to the GND.