Splitters and Hubs can do very much the same jobs IF the power limits are right. Both types of devices allow you to connect two or more fans to a single mobo fan header. When you do that, ALL fans on that connection device receive exactly the same power and control signals. IF those fans are identical, then their speeds and resulting air flows will be VERY close to each other. If the fans are different, then their performances will differ.
Recognize also that fans connected to DIFFERENT mobo fan header BUT fed the same signals (i.e., their two headers are configured identically) then those fans WILL match each other in performance IF they are identical fans. However, if they differ (for example, your situation with 140 mm fans on one header and 120 mm fans on another), then their performance will differ.
There is one vital factor in all this. Most mobo fan headers can provide power up to a max total load of 1.0 A per header. This directly impacts the choice if Splitter versus Hub. and WATCH OUT for the terms that sellers use for these devices - they often misuse the terms by treating them as meaning the same thing, and they are NOT. A SPLITTER is simple. It connects all its fans in parallel to the header, so the only source of power for those fans us what the header can provide. In using a Splitter, you MUST check the max current draw spec for each fan in the group and add them up. Most current fans max out at 0.10 to 0.25 A max current, so having three or four of these on ONE Splitter and header is OK. A HUB is a different device. The major difference is that it draws NO power for its fans from the mobo host header. Instead it uses an EXTRA connection "arm" to get power directly from the PSU via either a SATA power output or a 4-pin Molex output. These sources have much higher limits on max current available (4.5 A from SATA, more from Molex) so one rarely finds the total number of fans on a Hub exceeds the power limit. This is the easy way to connect many fans to a single header (without risking overloading the header's 1.0 A limit) and have them all receive the same signals. HOWEVER, almost all HUBS are designed to be able to control the speed ONLY of PWM 4-pin fans - they can NOT control the speed of older 3-pin fans. So if you have 3-pin fans, you can only use a Splitter for them. Hint on how to identify: a SPLITTER has ONE "arm" (or other input for certain designs) for input and two or more output "arms" for the fans. A HUB has those PLUS a third type of connection to the PSU for power.
OP, your plan is to keep on using 4 fans total - 2 140 mm front intakes and 2 120mm top and rear exhausts. None of those uses huge amps max. So you CAN do this with Splitters. You have one in use now, and you COULD (if you choose, but not necessary) use one more Splitter to connect both exhaust fans to ONE of your mobo headers.
One suggestion above was to upgrade to larger 140 mm fans for both the top and rear exhaust positions. In practice that would be almost impossible - the rear fan location, I am sure, cannot accept a 140 mm fan, although it may well be possible for the top. But there's another factor to consider that will impact what you choose. In setting up fans, one factor is the balance of air intake capacity versus air exhaust capacity. The background for this is that air flow capacity that is NOT equal (and that almost always happens) results in the air pressure inside your case being different from outside. If the internal pressure is marginally higher, then at any crack or leakage point the air will flow from inside to out. This is called "positive pressure" and means that no outside air containing dust can leak in at those points. The opposite unbalance (negative internal pressure) allows room air to leak in. Of course, to be effective as a dust preventive, this also means that the desired air intakes(your front fans) MUST have dust filters on them., and I full expect that your case has those. You just need to clean them from time to time. To achieve small positive pressure you can start with the count of fans in each function and their max air flow capacity ratings, although that won't be the final answer. Your current layout has two 140's front intake, two 120's exhaust. That is certainly more intake than exhaust capacity. Now add in this factor: the dust filters on the intakes actually reduce their capacity slightly, but they still will have more than your exhausts. IF you went to two 140's instead on the exhausts, the raw fan capacities would be equal, but the dust filters would make the real intakes slightly LESS than exhausts. You care not going to do that. But you are considering getting two new fans with HIGHER max airflow specs than your two old ones. Just check their ratings and add them up. That total should be a bit LESS than the total intake fan capacity.
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