There is a market for inserts/baffles for 4 inch air vents

It is quiet common for irish homes to have 4 inch holes in each room, these are called air vents and if you live in an exposed area you know all about them as they can be the source of significant heat loss.

I am not advocating their removal but I do think an insert that ensures that air flow passes/trickles through instead of howling through would make a compelling product.

I for one would buy it.