Great, that helps clarify. It leaves some questions unanswered, though.
First, since you have all four case fans now connected to mobo fan headers, I have to assume you found how to make those connections. MAYBE each of those fans has a standard female 3-pin fan connector on the end of its wires? Now, how many wires come out of each fan's motor to the connector? The "standard" case fan wiring has a Black wire to connector Pin #1 for Ground, a Red wire for DC supply voltage to Pin #2, and a Yellow wire for the speed signal to Pin #3. Is that what yours have, or something different?
I also note that the front fan, according to the case maker, has a switch to allow you to turn on or off its LED light. Where is that switch? Is it on a separate cord attached to the fan? FYI, may such fans that don't have any switch just feed the LED's from the DC supply the motor gets. Now, the motor uses up to 12 VDC, but the LED's only need 5 VDC for full brightness. So the LED's work just fine at most fan speeds, but get a bit dim when the fan is running very slow.
Related question: how do you plug 4 fans into 3 mobo SYS_FAN headers? Are you already using one Y-splitter? Or, is one of those plugged into the mobo CPU_OPT header? If that is the case, it's not quite ideal, but that's easy to change. You see, the SYS_FAN headers base their control on the temperature measured by a sensor in the mobo. The CPU_FAN and CPU_OPT headers base control instead on a sensor built into the CPU chip itself, so that system is not quite as good for CASE fan ventilation control as the SYS_FAN headers are. But to change is easy. If you install a 4-pin Hub, you can connect TWO of the current four case fans to that Hub as well as the four new ones. This make two case fans on two SYS_FAN headers, two plus the new four on the Hub, and the Hub has one SYS_FAN header to connect to.
Now, your concern about loading the headers etc. Your concern is well-founded. If you were to connect all those fans using splitters to the three SYS_FAN headers, you night risk overloading those headers. This is precisely why I said you MUST use a 4-pin HUB and not a splitter. A Hub draws all the power for its fans directly from the PSU through its connection to a SATA or Molex output. Thus it does NOT put any load on the mobo header it connects to. The header connection does only two functions: it returns one fan speed signal to the mobo, and it picks up the PWM signal from Pin #4 to share out to all its fans. The fan circuits that use that one control signal do not pull appreciable power from the mobo, so that does not overload the SYS_FAN header.
Now, we still have an important unanswered question: can the mobo SYS_FAN headers control the speed of your pre-installed case fans, or do they only run full speed all the time? The way I understand it, those pre-installed fans are very UNlikely to be 4-pin fans with 4 wires from their motors to their connectors. And 4-pin fans are the only type that your SYS_FAN headers can control. It sounds like you already have your system assembled and running, so do this observation carefully. Open your case and then start up the computer, observing those fans immediately. There are two possibilities for those case fans. IF they are under control, they all will start up at full speed for a few seconds, then slow down because the system temperature is low. After that as you do work, they may speed up a bit as temperatures rise. BUT what I really expect is that instead, they all start up at full speed and stay that way all the time. If that is the case, then those fans cannot be controlled by any 4-pin system using PWM control, and a different system is needed. So, post back what the fans really do.
IF those fans run full speed all the time, the "fix" is simple. I linked you to a Silverstone 4-pin Hub earlier, and hinted at a different one. There is another type called the Phanteks PWM Hub, here:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811984004&cm_re=Phanteks_PWM_Hub-_-11-984-004-_-Product
Like other Hubs it draws all power for its fans from a SATA output of the PSU, and connects to one mobo SYS_FAN header for its PWM control signal. But is has one difference that is important to you, OP. It does not merely share that PWM signal to 4-pin fans. It uses the signal internally to create its own group of 3-pin fan ports operating in Voltage Control Mode, which is what you need to control the speed of 3-pin (or 2-pin) fans. It has six ports on it, but like common mobo ports each of those can handle up to 1 amp load. So it CAN be used with Y-splitters to connect two fans to each port (except Port #1), so it can handle up to 11 fans that way. It even comes with two splitters, so it can handle eight fans out of the box.
This Phanteks PWM Hub makes use of a backwards compatibility feature in case fans. The connector and wiring of 3-pin and 4-pin fans are designed so that you can plug any female fan connector into any male output port and it will fit. The connectors have a tongue and groove molded into them so you can only plug them in the right way. Plugging a 3-pin fan into a 4-pin port gets you a fan that runs full speed all the time with no speed control. BUT plugging a 4-pin fan (your Noctuas) into a 3-pin port (on this Hub) DOES result in a fan running under full control. From an electrical perspective, this type of mis-match is not completely ideal, but the difference is small an does not matter for your use. So this Hub can power and control any mix of 3-pin and 4-pin fans.
Now, like any other 4-pin Hub, this Hub can only work if the mobo header it plugs into is truly a 4-pin header that operates in PWM Mode and thus provides a PWM signal on its Pin #4. Your mobo does it this way, and the proof of that will be in the speed test above: if your existing fans can only run at full speed off the mobo headers, they are true 4-pin headers. But the Phanteks instruction sheet tells you to do things a bit differently because some mobos do it oddly. They use headers with 4 pins, but operate them like 3-pin headers with a useless 4th pin, failing to provide the PWM signal. Just in case you have that type of mobo, the Phanteks instructions tell you to connect their Hub ONLY to the CPU_FAN header (which always is a true 4-pin type) and then connect your real CPU cooler to the Hub on Port #1. You should NOT do it that way. Leave your CPU cooling system connected to the CPU_FAN header. Plug the Phanteks Hub into one of the SYS_FAN headers because your mobo DOES provide the required PWM signal on Pin #4. Ensure that one of your new 4-pin Noctua fans is plugged into Port #1 (white) of the Hub so that it gets a fan speed signal to send back to the mobo. Then plug all the rest of your case fans - all the 3-pin pre-installed ones plus all the 4-pin Noctuas - into the Hub's ports, using a supplied splitter to get 7 fans on 6 ports. When you do this and observe the fans at start-up, they should all follow the pattern of starting fast, then slowing down.