Do I need both the 8 pin and 4 pin cpu power plug for my MSI B450 Pro Carbon?

From experience, it suffices to install the 8 pin connector on the left.

The right 4-pin-connector is basically a redundancy that the CPU may draw upon under extreme load most likely only relevant when you do extreme overclocking.

(The board manufacturers most likely also build in some more extra room in case the next CPU generation needs more power.)

So only connect the 2 ports (CPU_PWR_1) on the left, and leave the right one (CPU_PWR_2) empty. If your system won't boot or doesn't run stable, only then you should consider buying the extra cable or upgrading your PSU.