Your motherboard, the Asus ROG Maximus IX Hero, offers a total of 8 fan headers: CPU_FAN, CPU_OPT, CHA_FAN1, CHA_FAN2, CHA_FAN3, AIO_PUMP, W_PUMP+, and H_AMP.
The first 5 fan headers have a max. 1A current and is default as Q-Fan Controlled (variable speed). The AIO_PUMP header also has a max. 1A current but is default at Full Speed. The W_PUMP+ header offers up to max. 3A current and is also default at Full Speed. The H_AMP header also has a max. 3A current and is default as Q-Fan Controlled (variable speed).
Given these 8 fan headers and your intention to install 4 radiator fans and 3 exhaust case fans (total of 7), you are in no shortage of fan headers to connect to (including the AIO which uses a header for a grand total of 8).
Now, as to which fan headers to use is a matter of preference.
For me, I'd like to keep my non-variable AIO pump running at full speed all the time, so, I'd connect the H100i v2 on that AIO_PUMP header. I would also like my radiator fans to change their speed in consonance with the CPU temps. So, I'd get 2 PWM splitters (assuming you'll be using PWM radiator fans), and connect the first 2 PWM radiator fans on the CPU_FAN header and the second 2 PWM radiator fans on the CPU_OPT. These would work simultaneously.
For the 3 exhaust case fans, I would connect each one to the nearest CHA_FAN header.
I'd leave the W_PUMP+ and H_AMP headers not connected.