Here's my similar build which I made a few years ago, and still using it on (almost)daily basis(as you can see, they are pretty dusty
The switch in the middle of the block change the voltage of all four fans, between 7.5/15V(the fans are connected in two groups, each group with two fans in parallel; the switch change between the parallel/serial mode of the groups to 15V); the on/off switch is on the cable.
The power block(15V/3A) is from an old laptop, the fans(120X120X38mm) are coming from old rear-projection LCD TV's.
At 15V, it sound like hurricane, as expected, but it moves a LOTS of air; at 7.5V is pretty much silent.
The the fans are most efficient(move the most amount of air) mounted on a stand away(>10") from all surfaces, standing on surface(desk) are not that efficient.
If I would build a similar unit today, I would use Scythe Kaze Ultra 3000RPM fans, and I would build on a speaker stand with weight at the bottom, and OC, use the half/full switch. As power supply, I would use a 12-15V/3-5A brick.