Is this a high or a low number? On the one hand, <5% of units vacant sounds small. On the other hand, this is about 12x as many as there are estimated homeless in the US.
4% vacancy is what you get if people live in a house for two years before moving, and it takes a month before a new family moves in. Sounds pretty normal, and with no obvious way of using this idle time to help the homeless.
https://www.usich.gov/tools-for-action/2020-point-in-time-co...