DescentBB

So that's my question.

I have a case that uses 4 case fans, but my Mobo only has 3 case fan headers.
I already got planned a 3 fans layout (front, top and rear), just ordered the needed fans, but I noticed I will end with a spare fan, so I would like to see if it's possible to put it on the side, problem is I would run out of headers and would like the fan to be PWM controlled.

It's safe to use a splitter to connect 2 fans to a single header?
Or to begin with, is a side fan really needed if I already got the 3 fans I mentioned?

If it is a 4 pin splitter it will work fine, but a few things to keep in mind: It is generally not a good idea to mix different brands or even different models within the same brand fan on a splitter. The fans on a splitter should be exactly the same size, brand, model, RPM, etc. The splitter will report an RPM to the motherboard but that is the RPM of only one of the fans, the other fan will not be monitored, but both fans will operate on the same PWM signal so they will be running at the same duty cycle. Which is one of the primary reasons you don't want to mix different fans on the same channel, if the fan that isn't monitored has a higher PWM threshold it could stop entirely without you knowing. Also most motherboard headers are somewhat limited on the maximum power output, but case fans aren't generally high powered so it shouldn't be a problem. The typical limit is 1A and most case fans top off at 0.3 for the more powerful ones so a header can drive up to 3 with some room to spare. Just don't throw on a 120mm/4000 RPM Delta fan like the one I used to have that drew 3A.

For situations where you want more than just a couple fans, or more powerful fans, there are PWM splitting hubs that connect up as many as 8 fans and draw power from a SATA connector so it doesn't overload the motherboard header. I have one myself that I use to drive my 3 radiator fans, and I also have a 3 way Y splitting cable for my 3 case fans. Some day I'll get a case big enough to handle my radiator in full push+pull configuration and drive all 6 radiator fans from that single hub.

In the end I decided to connect the top fan directly to the PSU through Molex connector.

First of all, it's a big 200mm Cooler Master fan so it's pretty silent, and as it's a 3-pin one, so with Molex I get the full light brightness of the red LEDs and even if it's running at full speed, with it and the other 3 fans it barely sounds as louder as the previous Thermal Take fans I had on my previous case (that by the way were only 2 fans and were not working at full speed).