4,012. The Volume of Water; Remaining Overnight

Hilchos Bias HaMikdash 5:13

The kiyor must hold at least enough water for four kohanim to sanctify their hands and feet, as per Exodus 30:19, which says that Aaron and his sons shall sanctify from it. The kohanim of the time – Aaron, Elazar, Itamar and Pinchas – were four.

Hilchos Bias HaMikdash 5:14

The water in the kiyor is invalid if it remained overnight, as was discussed in Hilchos Beis HaBechirah. In order to prevent this, the kiyor was submerged in a mikvah or a spring and raised the next day. Alternatively, it would be filled each morning.