My understanding of these two fan designs differs somewhat from some of the posts above. The speed control of 3-pin fans is done solely by changing the voltage to the motor supplied on Pin #2, from 12 VDC for max speed down to about 5 VDC min for lowest speed. A lower voltage can stall the fan, and it won't re-start until that voltage is raised significantly. For a 4-pin fan, the voltage supply on Pin #2 is always the full 12 VDC, and it is NOT pulsed. (This type of "PWM fan" used in computer cases is NOT the same as DC motors commonly used in other applications with PWM motor drives. In those, the actual DC supply from drive to motor is pulsed. That is the reason that the makers of computer "PWM fans" tell you NOT to use them with a standard PWM drive system from other designs that actually would send pulsed power to the fan.) Within the computer PWM fan case there is a small chip that uses the PWM signal supplied on Pin #4 to modify the flow of current from the constant 12 VDC supply line through the motor windings, thus altering the average magnetic field force and changing the rotor speed.
The design of the newer 4-pin PWM fan system has a couple of backward compatibility features to ensure some success if you mix the two systems. First, the connectors and electrical signals are as similar as possible so that you CAN plug either fan type into either header type. If you connect a 3-pin fan to a 4-pin header that is using the new PWM Mode for control, the fan will receive a fixed 12 VDC supply from Pin #2 and will not receive the new PWM signal becasue it has no Pin #4. Besides, that fan has no special chip and could not modify current from the DC supply, anyway. So the fan will run full speed all the time with no harm other than it might wear out a little faster. You get good cooling - maybe too much - and higher noise than if the fan were able to slow down. The other mis-match is plugging a 4-pin fan into a 3-pin header that can only operate in Voltage Control Mode (aka DC Mode). That fan would receive no PWM signal becasue there is none and there is not even a 4th pin on the mobo header for it. It does receive from Pin #2 a DC supply that varies from 12 to 5 VDC, and that can NOT be modified by the chip in the casing because that chip gets no PWM signal. So the motor windings receive a NON-pulsed voltage which varies, and that makes it behave exactly as a 3-pin fan motor would. Its speed IS controlled by these signals, with no harm to the motor. The main impacts of this mis-match is that you lose two advanteges of the PWM design: they can be run at lower minimum speeds without stalling, and they can be started at a lower speed.
OP's original proposal is the correct way, I believe. That was to use SPLITTERS (not a HUB) to connect all four case fans to a single mobo case fan header, and ensure that the header is configured to use only Voltage Control Mode (aka DC Mode). That is the ONLY Mode that can control the speed of 3-pin fans. The backwards compatibility features inherent on the newer 4-pin fan design ensure that those fans also will be controlled by this power system. OP appears to have given correct consideration to an important limit in this proposal. Almost any fan header can supply up to 1.0 A max current to the fan(s) connected to it, and OP says the total load of the 4 fans is 0.68 A. As a hint, finding a four-output fan Splitter can be difficult, although I have seen ones that look like small printed circuit boards that can do this. The other simple option is to buy three 2-output Splitters and plug two of them into the outputs of the third. The resulting "stack" converts a single mobo fan header into four outputs.
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