Elwin's idea of just assuming you'll need a reduction and putting one in is a good one. It doesn't look like direct drive will give you enough power (overpropped).
Keep in mind that lower prop RPM is usually more efficient. I.e. the efficiency hit of a prop mostly comes from drag through the water, which goes up faster than linear. Keep RPM lower and pitch on the high side and you'll win compared to less pitch and more RPM (generally). The challenge with direct drive and your current motor is that your target prop speed of ~600mph is too low for your motor power-wise. This is actually a general challenge for electric boat motors, and why axial-flux motors are brought into play. If you end up needing a different motor, that'd be a good direction to consider (Lynch for example).
Once you have the ability to introduce nearly any reduction ratio you want to try, various combinations of battery voltage and reduction ratios can be experimented with. If 4kW can hit hull speed, it sounds like 24V and 2:1 reduction seems close to ideal.
As a final consideration, you may be very close to being able to hit a decent cruising speed (~6km/h) at full throttle. But, you usually want some headroom, and not run at maximum current 'all the time'. Wind/current/heavy-load will be more difficult to handle if you're at max power just cruising. So 24V / 2:1 seems like the obvious first thing to try.
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