So does having
four fans running sound reasonable - or unnecessary?

For me the more fans the better. I have seven hard drives in my video
editing computer which generates lots of heat.
If you count all my fans... there a
one in the power supply,
one on the processor,
two in the case rear,
one in the case front,
one on the video card,
one on the motherboard's chipset,
one in the side of the case

In case I missed any, ther are somewhere between 9 and 13 fans in my
tall tower case. It's noisy but necessary to keep it running cool.