As loadstar posted the Noctua NF-F12 PWM
would be a good choice for restricted spot, , according to noctua is good for rad/heatsinks and should be on a filter, usually is the fan Noctua includes in their 120mm coolers, it has been tested by SPCR on heatsink reviews like Noctua NH-U12S Slim Tower Heatsink
and Noctua NH-L12 Low Profile Cooler
, you could use a Akasa Flexa FP5 PWM 5-Way Splitter - Smart Fan Cable (AK-CBFA03-45)
to connect all 4 fans the same and control them on CPU_FAN header.
If you go the PWM route, you should pick your motherboard carefully, and ask yourself the following, do you want to control it via software like FanXpert2 or Speedfan (might or not support the sensor on the mobo), or pure bios. If its fanXpert2 then go with Asus motherboard (but be warned their bios pwm control is not that great in terms of allowing the user to pass 40%, on fanXpert is great). If you want bios fan control, then i strongly suggest you check go with MSI motherboard, their bios allows 12.5%, 25% and 37.5% all lower than Asus restrictions and to what i have seen Gigabyte aswel, not sure on Asrock though.
If you don't want PWM fans, the best alternative would be Scythe Gentle Typhoons, this has been the most popular fan among watercoolers/oveclocker for its high static preasure and the design of the bearing that will work fine on horizontal placement, there are lots of version on different RPMs, AP12/14 were reviewed by SPCR Fan Roundup #6: Scythe, Noiseblocker, Antec, Nexus, Thermalright
and came out really well, but the most commonly sold is Scythe Gentle Typhoon D1225C12B5AP-15 (1,850 rpm)
, to me at 12V is very loud, but the fan do likes undervolting, i'll try to run FanXpert2 on an AP15 if i can find the box where i store them, but you can also go for a lower rpm version, imo a good option could be the 1150rpm version, Scythe Gentle Typhoon D1225C12B3AP-13 (1,150 rpm)