Portable Power to 9 volts
The CN6009 is a boost converter, so the input voltage cannot be higher than 9 V. Therefore your choices for battery configuration are either 2S2P (series combination of 2 x 2 cells in parallel) giving 8.4 V max, or 4P (all four batteries in parallel) giving 4.2 V max.
The advantages of a simple parallel (3.7 V nominal) setup are:-

Batteries do not have to be precisely matched.

can use an odd number of cells.

Don't need a BMS (Battery Management System) to maintain cell balance.

Can use a commonly available 1 cell charging module and 5 V 'phone charger'
power supply.


Some likely disadvantages are:-

Lower boost converter efficiency due to higher input current.

Probably lower output current capability.

Long charging time due to limited current output of typical 1 cell chargers.


To use a 4 cell series battery (14.8 V nominal, 16.8 V max) you would need 4 matched cells, a 4 cell BMS (either with balancing, or combined with a sophisticated balancing charger), a 4 cell charger with suitable power supply, and a buck converter to drop the voltage to 9 V. This could be a bit more efficient, but also much more expensive and more complex.
For a 'bit of a noob' I recommend keeping it simple. Paralleling cells is easy and relatively safe (just make sure they have the same voltage when joined), and you may already have everything you need!